Merge branch 'main' of git.arirex.me:iris/ai-app

The drag-right-to-go-back gesture was removed on the other side while the
file explorer was being built on this one, so the explorer's own use of it
goes with it: back inside the explorer is the arrow and the platform's own
gesture, which is what the rest of the app now has too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -129,6 +129,26 @@ repo is in PLAN.md's "Backend layout" section.
   what makes two icon buttons the same width without either being given
   one, and it is why `GLYPH_SIZE` is smaller than it looks like it should
   be.
+- **The file explorer** — `FilesScreen.kt` (the navigation stack, the
+  per-directory cache, the create dialog), `FileViewer.kt` (a `LazyColumn`
+  of lines, each with its own colours from `FileLines.kt`, sharing one
+  horizontal scroll so nothing wraps), `FileEditor.kt` (a
+  `BasicTextField` with a `VisualTransformation` carrying the scanner's
+  spans, which is the one Compose API that colours a field's own text).
+  It draws **over** the session in `AppRoot`'s `Screen.Session`, so the
+  session under it stays composed and coming back from a file costs
+  nothing; back steps editor → viewer → directory → parent and only closes
+  from where it opened. `EXPLORER.md` is the design and `server/src/files.rs`
+  is the other half.
+  To exercise it, `./ui-sandbox.sh` builds a fixture tree at the sandbox
+  home's `~/files` holding the states that are otherwise only reachable by
+  finding a real machine in one: an empty directory, a name with a tab in
+  it and one with an apostrophe, a binary file, one over `FILE_LIMIT`, one
+  `chmod 000`, a symlink to a directory and a broken one, and a source file
+  per language. Point a session at it with
+  `./ui-sandbox.sh api /sessions/<id>/cwd -X POST -H 'content-type: application/json' -d '{"cwd":"~/files"}'`.
+  The 409 is produced by editing the file on the machine (`printf … > file`)
+  between pressing the pencil and pressing save.
 - `.dev-updater.ron` — what Dev Updater is asked to do with this checkout:
   the server (built in `server/`, run as `service: Managed(...)`) and the
   APK (built in `app/`), built in parallel. The project it serves is the
@@ -212,6 +232,23 @@ IQ2_XXS of that model produces fluent nonsense, which reads exactly like a
 broken driver — `llama-cli` produces the same from the file directly, which
 is how to tell the two apart in a hurry.
 
+**No script that drives this app's UI presses a coordinate.** Every control
+is found by the name it already carries for assistive technology --
+`ui-trace record --do "tap 'Session settings'"`, which resolves the label
+against the screen at the moment of the gesture and fails the whole run
+when it is not there. `app/bench-lib.sh` is what `transcript-bench.sh` and
+`stream-bench.sh` share for it. A coordinate is a position measured once by
+hand, and anything that moves the control makes the tap land on whatever
+now sits there -- the bench then reports a number that was never measured,
+which reads exactly like a result. Both scripts pressed the render report
+at `tap 723 205` until that button moved into the session settings dialog
+on 2026-09-03. The check that none has crept back:
+
+    grep -n "tap [0-9]" app/*.sh
+
+Swipes are still coordinates, deliberately: a gesture across a scrolling
+area is a distance rather than a control.
+
 **How to test SSH here, since there is no second machine:** ssh this VM to
 itself. Generate a throwaway key, append the public half to
 `~/.ssh/authorized_keys`, and configure a host of `bob@127.0.0.1` with

EXPLORER.md

@@ -7,10 +7,11 @@ existing syntax highlighting, line numbers, no wrapping; edit a file behind
 a pencil icon; create files through a modal like the ones the app already
 has; work over ssh; open at the session's working directory.
 
-This is the plan. Like PLAN.md it records each decision with the reason and
-what was rejected, so that when one changes it is changed here rather than
-re-argued. Once built, the operational notes (how to test it, what bit)
-move to AGENTS.md and this file keeps only the design.
+Built on 2026-09-03. This is the design, decision by decision with the
+reason and what was rejected, so that when one changes it is changed here
+rather than re-argued. The operational half -- how to run it, what to press,
+what to produce on purpose -- is in AGENTS.md, where the rest of this
+project's working notes are.
 
 ## What it is, in one paragraph
 
@@ -258,12 +259,18 @@ rerun and its output committed (it needs network):
   codepoint dev-updater uses, and it must not drift from it, as the cog
   and the refresh arrow already must not.
 - `md-plus` U+F0415 -- create. Also dev-updater's.
-- `md-pencil` -- edit.
-- `md-content_save` -- save.
-- `md-file_outline` -- file rows.
+- `md-pencil` U+F03EB -- edit.
+- `md-content_save` U+F0193 -- save.
+- `md-file_outline` U+F0224 -- file rows.
 
-The last three are verified against the Nerd Fonts cheat sheet when they
-are added, not copied from memory.
+All five were looked up in Nerd Fonts' own `glyphnames.json` rather than
+copied from memory, which is the check that a codepoint means the glyph its
+comment names.
+
+**Where the folder button sits**: between the usage chart and the cog, so
+the header reads widest scope to narrowest and the cog stays at the end
+where every other screen in this app keeps it. Asked for in that order by
+Iris on 2026-09-03.
 
 ### 13. The render report moves, and the benches move with it
 
@@ -386,32 +393,6 @@ pure functions with tests.
    `~/repos/emulator-tools`, `ui-trace`'s tap-by-label action; then the
    two bench scripts onto it, with no coordinate tap left in `app/*.sh`.
 
-## Testing
-
-- **Server**: `./run-tests.sh`, `cargo clippy --all-targets`, `cargo fmt`.
-- **Local transport, by hand**: `./ui-sandbox.sh api
-  "/setups/<id>/dir?path=~"` against the sandbox, whose `$HOME` is a
-  throwaway tree it is fine to write into. The sandbox gets a small
-  fixture directory with the states worth seeing: an empty directory, a
-  file with a tab in its name, a binary file, one over `FILE_LIMIT`, an
-  unreadable one (`chmod 000`), a symlink to a directory, and a source
-  file in each of a few languages.
-- **Remote transport**: the ssh-to-this-VM recipe in AGENTS.md ("How to
-  test SSH here"). The point of the exercise is the quoting and the
-  stdin path: write a file whose name has a `'` in it, and read it back.
-- **Phone**: `ui-trace`, not screenshots, for the things this feature is
-  made of -- that the gutter's number and its line share a baseline at
-  the first and the last row, that a long line's row is wider than the
-  viewport and does not grow the row height, that the editor's gutter
-  stays put while the text scrolls sideways. Screenshots for colour and
-  contrast on `rawSurface`.
-- **States to produce on purpose**, since the default state is the one
-  everybody looks at: a directory that fails to list (permission),
-  an unreachable machine (a setup pointing at a dead address), `binary`,
-  `tooBig`, the 409 conflict (edit the file with `sed -i` on the machine
-  between opening and saving), creating a name that exists, back with
-  unsaved edits, and the keyboard up over the editor.
-
 ## Numbers to measure, before deciding
 
 - Scan time for a 1 MiB source file on the emulator, and on the phone
@@ -425,19 +406,6 @@ pure functions with tests.
   If not, edit mode gets a lower cap than the viewer, stated in the
   editor rather than discovered by a stuck keyboard.
 
-## Order of work
-
-Each step leaves the app working and is one commit.
-
-1. Server: `files.rs` with `list` and `read`, routes, tests. Half a day.
-2. App: icons, `Api.kt`, `FilesScreen` listing, `FileViewer`, the root
-   and session wiring, the render-report move with the benches. A day.
-3. Server: `write`, `create_file`, `create_dir`, the stdin helper and
-   `ship_attachment` onto it. Half a day.
-4. App: `FileEditor`, the create dialog, the conflict dialog. Half a day.
-5. Measurements above, the sandbox fixture, PLAN.md and AGENTS.md. Half a
-   day.
-
 ## Later, deliberately not now
 
 - Delete, rename and move. Destructive controls belong here eventually,

app/androidApp/src/main/kotlin/com/example/aiapp/Api.kt

@@ -17,7 +17,18 @@ import org.json.JSONObject
 const val CONNECT_TIMEOUT_MS = 5000
 private const val READ_TIMEOUT_MS = 5000
 
-class ApiException(message: String, cause: Throwable? = null) : Exception(message, cause)
+/**
+ * A request that did not produce what it asked for, carrying the server's own wording where it sent
+ * some -- those messages are written to be read on the screen that made the call.
+ *
+ * [status] is the HTTP status where there was a response at all, and null where the server was
+ * never reached. Callers that need it need it because the *same* failure is two different things to
+ * do: a 409 from a write is "somebody else changed this, here are three ways out", where every
+ * other refusal is a message to show. Nothing should branch on it to decide what to *say* -- the
+ * message is what says that.
+ */
+class ApiException(message: String, val status: Int? = null, cause: Throwable? = null) :
+    Exception(message, cause)
 
 /**
  * Runs one request against the backend, with the pinned TLS setup, the bearer token, and the
@@ -68,7 +79,8 @@ fun <T> requestFromServer(
                     detail.isNullOrEmpty() ->
                         "Server returned HTTP ${connection.responseCode} for $path"
                     else -> detail
-                }
+                },
+                status = connection.responseCode,
             )
         }
         return readBody(connection)
@@ -82,13 +94,13 @@ fun <T> requestFromServer(
             "Couldn't reach the server at ${settings.baseUrl} " +
                 "(${e::class.simpleName}: ${e.message}) -- is ai-server running, and is " +
                 "this device able to reach that address (WireGuard up)?",
-            e,
+            cause = e,
         )
     } catch (e: Exception) {
         throw ApiException(
             "Reached ${settings.baseUrl}$path but couldn't read its response " +
                 "(${e::class.simpleName}: ${e.message})",
-            e,
+            cause = e,
         )
     } finally {
         connection.disconnect()
@@ -589,6 +601,175 @@ fun uploadAttachment(
     }
 }
 
+/**
+ * One entry of a directory on the machine a setup names.
+ *
+ * [kind] is the *target's* where the entry is a symlink, so a link to a directory descends; [link]
+ * still says it is one. Neither is worked out here -- the machine answers both, because it is the
+ * only thing that can.
+ */
+data class DirEntry(
+    val name: String,
+    val kind: String,
+    val size: Long,
+    val modified: Long,
+    val link: Boolean,
+) {
+    val isDirectory: Boolean
+        get() = kind == "directory"
+}
+
+/** A directory's entries, and the path the machine resolved the request to. */
+data class Listing(val path: String, val entries: List<DirEntry>)
+
+/**
+ * What reading a file produced.
+ *
+ * Four cases, because they are four different things to draw and none of them is an error the
+ * screen can shrug off: content, something that is not text, something too big to have sent, and
+ * (as [ApiException], not a case here) the machine's own refusal. A file with nothing in it is
+ * [FileContent.Text] with an empty string -- which is what it is, and not the same as any of these.
+ */
+sealed class FileContent {
+    abstract val path: String
+    abstract val size: Long
+    abstract val modified: Long
+
+    data class Text(
+        override val path: String,
+        override val size: Long,
+        override val modified: Long,
+        /** What a write is given back, to prove the file is still the one that was read. */
+        val sha256: String,
+        val content: String,
+    ) : FileContent()
+
+    data class Binary(
+        override val path: String,
+        override val size: Long,
+        override val modified: Long,
+    ) : FileContent()
+
+    data class TooBig(
+        override val path: String,
+        override val size: Long,
+        override val modified: Long,
+    ) : FileContent()
+}
+
+/** What a file is after a write, so the editor's precondition is fresh without a second read. */
+data class Written(val size: Long, val modified: Long, val sha256: String)
+
+/** Everything in [path] on the machine [setup] names, and what [path] resolved to. */
+fun fetchDir(settings: ServerSettings, setup: String, path: String): Listing =
+    requestFromServer(
+        settings,
+        "/setups/${setup.urlEncoded()}/dir?path=${path.urlEncoded()}",
+        readTimeoutMs = 30000,
+    ) { connection ->
+        val body = connection.jsonObject()
+        Listing(
+            path = body.getString("path"),
+            entries =
+                body.getJSONArray("entries").mapObjects { entry ->
+                    DirEntry(
+                        name = entry.getString("name"),
+                        kind = entry.getString("kind"),
+                        size = entry.optLong("size"),
+                        modified = entry.optLong("modified"),
+                        link = entry.optBoolean("link", false),
+                    )
+                },
+        )
+    }
+
+/** One file's content, or which of the reasons there is none to show. */
+fun fetchFile(settings: ServerSettings, setup: String, path: String): FileContent =
+    requestFromServer(
+        settings,
+        "/setups/${setup.urlEncoded()}/file?path=${path.urlEncoded()}",
+        // A megabyte over the tunnel, and a `stat` plus a `sha256sum` on the far machine before
+        // any of it moves. Well clear of that rather than just above it -- a timeout is for a
+        // server that has stopped answering.
+        readTimeoutMs = 60000,
+    ) { connection ->
+        val body = connection.jsonObject()
+        val at = body.getString("path")
+        val size = body.optLong("size")
+        val modified = body.optLong("modified")
+        when (val kind = body.getString("kind")) {
+            "text" ->
+                FileContent.Text(
+                    at,
+                    size,
+                    modified,
+                    body.getString("sha256"),
+                    body.getString("content"),
+                )
+            "binary" -> FileContent.Binary(at, size, modified)
+            "tooBig" -> FileContent.TooBig(at, size, modified)
+            // A backend that has learned a fifth answer. Reported rather than guessed at: picking
+            // the nearest of the four would draw something confident about a state this app has
+            // never seen.
+            else ->
+                throw ApiException(
+                    "The server described this file as \"$kind\", which this app does not know how to show."
+                )
+        }
+    }
+
+/**
+ * Replaces a file's contents, but only while it still hashes to [ifSha256].
+ *
+ * The refusal is a 409 and arrives as an [ApiException] carrying the server's wording, which is
+ * what the conflict dialog shows -- an agent editing the same file while somebody reads it is the
+ * ordinary case here, not the exotic one.
+ */
+fun writeFile(
+    settings: ServerSettings,
+    setup: String,
+    path: String,
+    content: String,
+    ifSha256: String,
+): Written =
+    requestFromServer(
+        settings,
+        "/setups/${setup.urlEncoded()}/file",
+        method = "PUT",
+        jsonBody =
+            JSONObject()
+                .put("path", path)
+                .put("content", content)
+                .put("ifSha256", ifSha256)
+                .toString(),
+        readTimeoutMs = 60000,
+    ) { connection ->
+        val body = connection.jsonObject()
+        Written(body.optLong("size"), body.optLong("modified"), body.getString("sha256"))
+    }
+
+/** Creates an empty file. Refused, with the machine's own words, if the name is already taken. */
+fun createFile(settings: ServerSettings, setup: String, path: String) {
+    requestFromServer(
+        settings,
+        "/setups/${setup.urlEncoded()}/file",
+        method = "POST",
+        jsonBody = JSONObject().put("path", path).toString(),
+        readTimeoutMs = 30000,
+    ) {}
+}
+
+/** Creates a directory, with the same refusal as [createFile]. */
+fun createDir(settings: ServerSettings, setup: String, path: String) {
+    requestFromServer(
+        settings,
+        "/setups/${setup.urlEncoded()}/dir",
+        method = "POST",
+        jsonBody = JSONObject().put("path", path).toString(),
+        readTimeoutMs = 30000,
+    ) {}
+}
+
 /** Fetches an image the transcript references (produced or uploaded). */
 fun fetchSessionFile(settings: ServerSettings, sessionId: String, name: String): ByteArray =
     requestFromServer(settings, "/sessions/$sessionId/files/$name", readTimeoutMs = 30000) {

app/androidApp/src/main/kotlin/com/example/aiapp/AppRoot.kt

@@ -38,7 +38,17 @@ import kotlinx.coroutines.withContext
 private sealed class Screen {
     data object Main : Screen()
 
-    data class Session(val summary: SessionSummary) : Screen()
+    /**
+     * One session, with the file explorer over it when [files] is set.
+     *
+     * The explorer is a layer on this screen rather than a screen of its own, so the session under
+     * it stays composed: its event stream keeps flowing, its scroll position and draft stay put,
+     * and coming back from a file costs nothing. As a sibling `Screen` it would be disposed and
+     * re-created on every return, refetching the transcript over the tunnel -- which is exactly the
+     * flip between "what did it change" and "what is it saying" that this feature exists for. The
+     * image viewer already made the same choice for the same reason.
+     */
+    data class Session(val summary: SessionSummary, val files: FilesTarget? = null) : Screen()
 
     data object Spawn : Screen()
 
@@ -198,14 +208,28 @@ fun AppRoot(
             // row key. Only reachable since a notification can move straight from one session to
             // another; every other way here passes through [Screen.Main], which disposes it anyway.
             key(here.summary.id) {
-                // No imePadding here, for the reason above.
-                SessionScreen(
-                    settings = current,
-                    summary = here.summary,
-                    onBack = goToMain,
-                    share = share,
-                    onShareTaken = { share = null },
-                )
+                // A Box so the explorer can be drawn *over* the session rather than instead of
+                // it; the session stays composed underneath. No imePadding here, for the reason
+                // above -- the explorer adds its own, since it has a text field.
+                Box {
+                    SessionScreen(
+                        settings = current,
+                        summary = here.summary,
+                        onBack = goToMain,
+                        onFiles = { screen = here.copy(files = it) },
+                        share = share,
+                        onShareTaken = { share = null },
+                    )
+                    // Its own back handler is registered after this screen's, so it is the one the
+                    // platform asks first, and it steps back inside itself before closing.
+                    here.files?.let { target ->
+                        FilesScreen(
+                            settings = current,
+                            target = target,
+                            onClose = { screen = here.copy(files = null) },
+                        )
+                    }
+                }
             }
         is Screen.Spawn ->
             Box(Modifier.imePadding()) {

app/androidApp/src/main/kotlin/com/example/aiapp/Attachments.kt

@@ -64,7 +64,7 @@ suspend fun uploadPicked(
         } catch (e: java.io.IOException) {
             // Either side of the copy can fail; the message names the file, which is the
             // part the reader can do something about.
-            throw ApiException("couldn't send $name: ${e.message}", e)
+            throw ApiException("couldn't send $name: ${e.message}", cause = e)
         }
     }
 }

app/androidApp/src/main/kotlin/com/example/aiapp/CodeFence.kt

@@ -137,6 +137,25 @@ private fun CodeBlockText(
 fun fenceLanguage(name: String?): Language? =
     FENCE_LANGUAGES[name?.trim()?.lowercase() ?: return null]
 
+/**
+ * The highlighter's language for a *file*, from its name.
+ *
+ * The same table [fenceLanguage] reads, deliberately: it already keys on the extensions people
+ * write after the backticks -- `kt`, `rs`, `py` -- because the extension is as often what gets
+ * written there as the language's name. One table rather than two, so a language added for fences
+ * is a language added for files and neither can be the one somebody forgot.
+ *
+ * The extension is the part after the *last* dot, which is what makes `build.gradle.kts` Kotlin and
+ * `Cargo.toml` TOML. A leading dot is not one: `.bashrc` has no extension, it has a name that
+ * starts with a dot, and reading `bashrc` as an extension would look up a word no table has. A name
+ * with no dot at all -- `Makefile`, `LICENSE` -- is likewise null, and null is drawn plain.
+ */
+fun fileLanguage(name: String): Language? {
+    val dot = name.lastIndexOf('.')
+    if (dot < 1) return null
+    return fenceLanguage(name.substring(dot + 1))
+}
+
 private val FENCE_LANGUAGES: Map<String, Language> =
     mapOf(
         "kotlin" to Language.KOTLIN,

app/androidApp/src/main/kotlin/com/example/aiapp/FileEditor.kt

@@ -0,0 +1,143 @@
+package com.example.aiapp
+
+import androidx.compose.foundation.horizontalScroll
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.text.BasicTextField
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.SolidColor
+import androidx.compose.ui.text.input.OffsetMapping
+import androidx.compose.ui.text.input.TextFieldValue
+import androidx.compose.ui.text.input.TransformedText
+import androidx.compose.ui.text.input.VisualTransformation
+import androidx.compose.ui.text.style.TextAlign
+
+/**
+ * The same file, editable, in the same face and colours it was being read in.
+ *
+ * `BasicTextField(TextFieldValue)` with a [VisualTransformation] is the one Compose arrangement
+ * that colours a field's own text rather than replacing the field with something that only looks
+ * like one: the transformation returns the text unchanged and the scanner's spans as styles, so
+ * [OffsetMapping.Identity] is correct by construction -- no character moves, so no offset does. The
+ * newer `TextFieldState` API has no hook for styles at all, which is why this is the older one.
+ *
+ * The cost is that the whole file is re-scanned on every keystroke. For a file under the server's
+ * limit that is expected to be a few milliseconds; see EXPLORER.md's "Numbers to measure", which is
+ * where a size below which highlighting is switched off would be decided if it turns out to be
+ * needed.
+ *
+ * The gutter is one `Text` of `1\n2\n…` beside the field rather than a number per row, because
+ * there are no rows here -- the field is one text object. It stays put while the text scrolls
+ * sideways, and it lines up for the same reason the viewer's does: nothing wraps, so a logical line
+ * is a visual line.
+ */
+@Composable
+fun FileEditor(
+    value: TextFieldValue,
+    onValueChange: (TextFieldValue) -> Unit,
+    language: Language?,
+    modifier: Modifier = Modifier,
+) {
+    val style = codeStyle().copy(color = MaterialTheme.colorScheme.onSurface)
+    val scroll = rememberScrollState()
+    val count = value.text.removeSuffix("\n").count { it == '\n' } + 1
+    val gutter = gutterWidth(count, style)
+    val numbers = remember(count) { (1..count).joinToString("\n") }
+    val transformation =
+        remember(language) {
+            VisualTransformation { text ->
+                TransformedText(highlight(text.text, language), OffsetMapping.Identity)
+            }
+        }
+    Row(verticalAlignment = Alignment.Top, modifier = modifier.fillMaxWidth()) {
+        Text(
+            numbers,
+            style = style,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+            textAlign = TextAlign.End,
+            softWrap = false,
+            modifier = Modifier.width(gutter),
+        )
+        // The same gap the viewer puts between its numbers and its code, so switching between
+        // reading and editing does not move the text sideways under the reader.
+        Spacer(Modifier.width(GUTTER_GAP))
+        Box(Modifier.horizontalScroll(scroll)) {
+            BasicTextField(
+                value = value,
+                onValueChange = onValueChange,
+                textStyle = style,
+                cursorBrush = SolidColor(MaterialTheme.colorScheme.primary),
+                visualTransformation = transformation,
+            )
+        }
+    }
+}
+
+/**
+ * What to do about a file that changed on the machine while it was open here.
+ *
+ * Three ways out rather than one, and each says what it costs, because there is no answer this app
+ * can pick on somebody's behalf: an agent editing the same file is the ordinary case here, and both
+ * versions are somebody's work.
+ */
+@Composable
+fun ConflictDialog(
+    message: String,
+    busy: Boolean,
+    onOverwrite: () -> Unit,
+    onReload: () -> Unit,
+    onCancel: () -> Unit,
+) {
+    AlertDialog(
+        onDismissRequest = onCancel,
+        // The server's own sentence as the title, rather than a heading of this app's above it
+        // saying the same thing twice: there is one statement of what happened and it comes from
+        // the side that found out.
+        title = { Text(message.replaceFirstChar { it.uppercase() }) },
+        text = {
+            Text(
+                "Overwrite keeps what you typed and loses the other change. " +
+                    "Reload keeps the other change and loses what you typed. " +
+                    "Cancel leaves both alone and keeps you here."
+            )
+        },
+        confirmButton = {
+            TextButton(onClick = onOverwrite, enabled = !busy) {
+                Text(if (busy) "Saving..." else "Overwrite")
+            }
+        },
+        dismissButton = {
+            Row {
+                TextButton(onClick = onReload, enabled = !busy) { Text("Reload") }
+                TextButton(onClick = onCancel, enabled = !busy) { Text("Cancel") }
+            }
+        },
+    )
+}
+
+/** Leaving an editor with edits in it, which is the one way to lose them by accident. */
+@Composable
+fun UnsavedDialog(onDiscard: () -> Unit, onCancel: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onCancel,
+        title = { Text("Leave without saving?") },
+        text = {
+            Text(
+                "The edits you have made here will be lost. They have not been written to the machine."
+            )
+        },
+        confirmButton = { TextButton(onClick = onDiscard) { Text("Discard") } },
+        dismissButton = { TextButton(onClick = onCancel) { Text("Keep editing") } },
+    )
+}

app/androidApp/src/main/kotlin/com/example/aiapp/FileLines.kt

@@ -0,0 +1,104 @@
+package com.example.aiapp
+
+import androidx.compose.ui.text.AnnotatedString
+import androidx.compose.ui.text.SpanStyle
+import androidx.compose.ui.text.buildAnnotatedString
+
+/**
+ * A file split into lines, with the highlighter's colours already worked out for each one.
+ *
+ * The pure half of the viewer, so it has a JVM unit test and so [of] can run off the main thread:
+ * scanning a megabyte is work, and doing it inside a composable would do it on the drawing thread
+ * and again on every recomposition.
+ *
+ * Why per line at all: the viewer is a `LazyColumn` of lines rather than one `Text`, because text
+ * layout is linear in the text and a twenty-thousand-line file in one `Text` measures all of it to
+ * draw a screenful. That means each row needs *its* colours, and the scanner answers in offsets
+ * into the whole file -- so the spans are bucketed here, once, in one pass over an already-ordered
+ * list, rather than each row searching the whole list for the part that is its.
+ */
+class FileLines
+private constructor(
+    /** The text of each line, without its newline. */
+    val lines: List<String>,
+    /** Per line, the spans that fall in it, with offsets relative to that line's start. */
+    private val spans: List<List<Span>>,
+) {
+    val size: Int
+        get() = lines.size
+
+    /**
+     * One line, coloured.
+     *
+     * Built when the row is composed rather than up front: a file has far more lines than a screen
+     * shows, and an `AnnotatedString` per line for all of them is the cost the lazy list exists to
+     * avoid.
+     */
+    fun line(index: Int): AnnotatedString {
+        val text = lines[index]
+        val here = spans[index]
+        if (here.isEmpty()) return AnnotatedString(text)
+        val palette = catppuccinSyntax()
+        return buildAnnotatedString {
+            append(text)
+            here.forEach { addStyle(SpanStyle(color = palette.of(it.kind)), it.start, it.end) }
+        }
+    }
+
+    companion object {
+        /**
+         * [text] scanned as [language] and cut into lines.
+         *
+         * Exactly one trailing newline is dropped before splitting, so a file that ends the way
+         * text files are supposed to end has the number of lines its author would count -- `wc -l`
+         * agrees, and so does every editor. Without that, every well-formed file gained a phantom
+         * empty last line, which is a wrong line number on every file in the repository. An empty
+         * file is one empty line numbered 1, which is what it is: a file with nothing in it still
+         * has somewhere for a cursor to go.
+         */
+        fun of(text: String, language: Language?): FileLines {
+            val body = text.removeSuffix("\n")
+            val lines = body.split('\n')
+            val rules = language?.let { rulesOf(it) }
+            val scanned = if (rules == null) emptyList() else scan(body, rules)
+            return FileLines(lines, bucket(lines, scanned))
+        }
+
+        /**
+         * The scanner's spans, in file offsets, as spans per line in line offsets.
+         *
+         * One walk down both lists, which is what the scanner's guarantee buys: its spans come out
+         * ordered, non-overlapping and inside the text, so a span can only belong to the line the
+         * walk has reached or to ones after it. A span crossing a line break -- a block comment, a
+         * multi-line string -- is cut at each break and appears in each line it covers, because a
+         * row is drawn on its own and cannot inherit a colour from the row above.
+         */
+        private fun bucket(lines: List<String>, spans: List<Span>): List<List<Span>> {
+            val out = ArrayList<List<Span>>(lines.size)
+            var lineStart = 0
+            var next = 0
+            for (line in lines) {
+                val lineEnd = lineStart + line.length
+                var here: ArrayList<Span>? = null
+                // Spans that ended before this line begins are behind the walk for good.
+                while (next < spans.size && spans[next].end <= lineStart) next++
+                var at = next
+                while (at < spans.size && spans[at].start < lineEnd) {
+                    val span = spans[at]
+                    val start = maxOf(span.start, lineStart) - lineStart
+                    val end = minOf(span.end, lineEnd) - lineStart
+                    if (end > start) {
+                        (here ?: ArrayList<Span>().also { here = it }).add(
+                            Span(start, end, span.kind)
+                        )
+                    }
+                    at++
+                }
+                out.add(here ?: emptyList())
+                // The newline itself, which is in the text and not in any line.
+                lineStart = lineEnd + 1
+            }
+            return out
+        }
+    }
+}

app/androidApp/src/main/kotlin/com/example/aiapp/FileViewer.kt

@@ -0,0 +1,119 @@
+package com.example.aiapp
+
+import androidx.compose.foundation.horizontalScroll
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.text.selection.SelectionContainer
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.text.AnnotatedString
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.font.FontFamily
+import androidx.compose.ui.text.rememberTextMeasurer
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+
+/** The face every verbatim thing in this app is drawn in, and the one the gutter has to match. */
+@Composable
+fun codeStyle(): TextStyle =
+    MaterialTheme.typography.bodySmall.copy(fontFamily = FontFamily.Monospace)
+
+/**
+ * A file, one line per row, coloured by the same scanner that colours a reply's code fences.
+ *
+ * A `LazyColumn` of lines rather than one `Text`, because text layout is linear in the text: a
+ * twenty-thousand-line file in a single `Text` measures all of it to draw a screenful, and the
+ * scroll never recovers. The cost of the choice is that each row needs its own colours, which is
+ * what [FileLines] works out once and off this thread.
+ *
+ * Lines do not wrap. They share one horizontal scroll state, so the whole file moves sideways as a
+ * block and a long line does not silently become three -- which would put the gutter's numbers
+ * against the wrong text, the one thing a numbered listing must never do. Because nothing wraps, a
+ * logical line is one visual line and the two cannot drift.
+ *
+ * The gutter stays put while the text scrolls, so a line number is still there to read at the right
+ * hand end of a long line. Its width is measured from the digit count of the line count in the very
+ * style it is drawn in, so a nine-line file and a twelve-thousand-line file each get exactly what
+ * they need and nothing is nudged by hand.
+ */
+@Composable
+fun FileViewer(lines: FileLines, modifier: Modifier = Modifier) {
+    val style = codeStyle()
+    val scroll = rememberScrollState()
+    val gutter = gutterWidth(lines.size, style)
+    // One container around the whole file rather than one per line, so a selection can run across
+    // lines -- the same arrangement the transcript uses.
+    SelectionContainer(modifier) {
+        LazyColumn(Modifier.fillMaxWidth()) {
+            items(lines.size) { index ->
+                Row(verticalAlignment = Alignment.Top) {
+                    LineNumber(index + 1, gutter, style)
+                    Text(
+                        lines.line(index),
+                        style = style,
+                        softWrap = false,
+                        modifier = Modifier.horizontalScroll(scroll),
+                    )
+                }
+            }
+        }
+    }
+}
+
+/**
+ * One line's number, right-aligned in the gutter.
+ *
+ * `onSurfaceVariant`, because it is not part of the file: it is this app numbering it, and giving
+ * it the text's own colour would put it in the same voice as the code.
+ */
+@Composable
+fun LineNumber(number: Int, width: Dp, style: TextStyle) {
+    Text(
+        number.toString(),
+        style = style,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        textAlign = TextAlign.End,
+        maxLines = 1,
+        modifier = Modifier.width(width),
+    )
+    Spacer(Modifier.width(GUTTER_GAP))
+}
+
+/**
+ * How wide the widest line number is, measured rather than guessed.
+ *
+ * `9` repeated, because digits in a monospace face are all one width and the count's own digits
+ * would measure the same -- what matters is how many there are. Measuring in the style the numbers
+ * are drawn in is what makes this survive a font size, a density or a display scale nobody here
+ * chose.
+ */
+@Composable
+fun gutterWidth(lineCount: Int, style: TextStyle): Dp {
+    val measurer = rememberTextMeasurer()
+    val density = LocalDensity.current
+    val digits = maxOf(1, lineCount.toString().length)
+    return remember(digits, style, density) {
+        with(density) {
+            measurer.measure(AnnotatedString("9".repeat(digits)), style).size.width.toDp()
+        }
+    }
+}
+
+/**
+ * The space between the numbers and the code.
+ *
+ * A gap, not an alignment: the two are already aligned by the row, and this is only so the digits
+ * and the first character of the line are not touching.
+ */
+val GUTTER_GAP = 8.dp

app/androidApp/src/main/kotlin/com/example/aiapp/FilesScreen.kt

@@ -0,0 +1,678 @@
+package com.example.aiapp
+
+import androidx.activity.compose.BackHandler
+import androidx.compose.foundation.background
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.ColumnScope
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.imePadding
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.CircularProgressIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Switch
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateMapOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.input.TextFieldValue
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+
+/**
+ * Which machine's files to show, and where to start.
+ *
+ * A **setup**, not a session: a filesystem is a property of a machine, and a session only says
+ * where it was working. That is what makes a second way in -- from the setups tab, say -- one more
+ * caller rather than any new code here.
+ */
+data class FilesTarget(val setup: String, val setupName: String, val start: String)
+
+/** Where the explorer is: in a directory, or in one file. */
+private sealed class Spot(val path: String) {
+    class Dir(path: String) : Spot(path)
+
+    class Doc(path: String) : Spot(path)
+}
+
+/**
+ * The files on the machine a session runs on: browse them, read one, change one.
+ *
+ * Drawn **over** the session rather than instead of it (see [AppRoot]), so its event stream keeps
+ * flowing, its draft and scroll position stay where they were, and coming back from a file costs
+ * nothing. Back steps one level inside here -- editor to viewer, viewer to the directory it came
+ * from, directory to the one above it -- and only closes from where it opened.
+ *
+ * Every directory that has been visited is kept for as long as this is open, so stepping back is
+ * instant; the refresh glyph is how a directory gets asked again on purpose, and creating something
+ * refetches the directory it was created in, since that is the one thing that changed.
+ */
+@Composable
+fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Unit) {
+    val scope = rememberCoroutineScope()
+    var stack by remember { mutableStateOf(listOf<Spot>(Spot.Dir(target.start))) }
+    val listings = remember { mutableStateMapOf<String, LoadState<Listing>>() }
+    var creating by remember { mutableStateOf(false) }
+    // Edit mode and whether anything has been typed live here rather than in the pane below,
+    // because they are what back has to know about -- and back arrives from two places, the arrow
+    // and the platform's own gesture, which must mean the same thing.
+    var editing by remember { mutableStateOf(false) }
+    var dirty by remember { mutableStateOf(false) }
+    var askUnsaved by remember { mutableStateOf(false) }
+
+    val here = stack.last()
+
+    fun go(spot: Spot) {
+        editing = false
+        dirty = false
+        stack = stack + spot
+    }
+
+    fun back() {
+        when {
+            editing && dirty -> askUnsaved = true
+            editing -> editing = false
+            stack.size > 1 -> {
+                stack = stack.dropLast(1)
+                editing = false
+                dirty = false
+            }
+            else -> onClose()
+        }
+    }
+
+    suspend fun load(path: String, again: Boolean) {
+        if (!again && listings[path] is LoadState.Loaded) return
+        listings[path] = LoadState.Loading
+        listings[path] =
+            try {
+                withContext(Dispatchers.IO) {
+                    LoadState.Loaded(fetchDir(settings, target.setup, path))
+                }
+            } catch (e: ApiException) {
+                LoadState.failed(e)
+            }
+    }
+
+    BackHandler(onBack = ::back)
+
+    Box(
+        Modifier.fillMaxSize()
+            .background(MaterialTheme.colorScheme.background)
+            // The session under this deliberately takes no keyboard inset (see SessionScreen's
+            // layout note), so the explorer adds its own -- otherwise the editor types under the
+            // keyboard.
+            .imePadding()
+    ) {
+        Column(Modifier.fillMaxSize()) {
+            when (val spot = here) {
+                is Spot.Dir -> {
+                    val state = listings[spot.path] ?: LoadState.Loading
+                    // The resolved path once there is one: a directory opened as `~` is called
+                    // what it turned out to be, not what it was asked for.
+                    val at = (state as? LoadState.Loaded)?.value?.path ?: spot.path
+                    FilesHeader(
+                        title = baseName(at),
+                        path = at,
+                        machine = target.setupName,
+                        onBack = ::back,
+                    ) {
+                        GlyphButton(
+                            REFRESH_GLYPH,
+                            "Refresh this directory",
+                            { scope.launch { load(spot.path, again = true) } },
+                            enabled = state !is LoadState.Loading,
+                        )
+                        GlyphButton(
+                            PLUS_GLYPH,
+                            "Create here",
+                            { creating = true },
+                            enabled = state is LoadState.Loaded,
+                        )
+                    }
+                    LaunchedEffect(spot.path) { load(spot.path, again = false) }
+                    DirectoryBody(state, onOpen = ::go)
+                }
+                is Spot.Doc ->
+                    DocPane(
+                        settings = settings,
+                        target = target,
+                        path = spot.path,
+                        name = baseName(spot.path),
+                        editing = editing,
+                        onEditing = { editing = it },
+                        onDirty = { dirty = it },
+                        onBack = ::back,
+                    )
+            }
+        }
+    }
+
+    if (askUnsaved) {
+        UnsavedDialog(
+            onDiscard = {
+                askUnsaved = false
+                editing = false
+                dirty = false
+            },
+            onCancel = { askUnsaved = false },
+        )
+    }
+
+    val dir = here as? Spot.Dir
+    val listing = (listings[dir?.path] as? LoadState.Loaded)?.value
+    if (creating && dir != null && listing != null) {
+        CreateDialog(
+            settings = settings,
+            setup = target.setup,
+            directory = listing.path,
+            onDismiss = { creating = false },
+            onCreated = { path, isDirectory ->
+                creating = false
+                scope.launch {
+                    // The directory it was created in is the one thing that changed, so that is
+                    // what gets asked again -- not the whole stack.
+                    load(dir.path, again = true)
+                    // A new file has nothing to look at, so it opens where it can be filled in.
+                    if (!isDirectory) {
+                        go(Spot.Doc(path))
+                        editing = true
+                    }
+                }
+            },
+        )
+    }
+}
+
+/**
+ * The row every view in here has at the top: back, what this is, and what acts on it.
+ *
+ * The path is truncated in the middle when it will not fit, because both ends carry something the
+ * reader needs -- the machine and the top of the tree at one end, the file at the other -- and it
+ * is the longest paths, the ones being read most closely, that get cut.
+ */
+@Composable
+private fun FilesHeader(
+    title: String,
+    path: String,
+    machine: String,
+    onBack: () -> Unit,
+    actions: @Composable () -> Unit,
+) {
+    Row(
+        verticalAlignment = Alignment.CenterVertically,
+        modifier = Modifier.fillMaxWidth().padding(horizontal = 16.dp),
+    ) {
+        GlyphButton(BACK_GLYPH, "Back", onBack)
+        Spacer(Modifier.width(GLYPH_BUTTON_MARGIN))
+        Column(Modifier.weight(1f)) {
+            Text(title, style = MaterialTheme.typography.titleMedium, maxLines = 1)
+            Text(
+                "$machine · $path",
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                maxLines = 1,
+                overflow = TextOverflow.MiddleEllipsis,
+            )
+        }
+        Row { actions() }
+    }
+}
+
+/**
+ * What is in a directory.
+ *
+ * A listing that failed says why, in the machine's own words, where the rows would be -- never an
+ * empty list, which is what "there is nothing here" looks like and is the one wrong answer that
+ * looks like a right one.
+ */
+@Composable
+private fun ColumnScope.DirectoryBody(state: LoadState<Listing>, onOpen: (Spot) -> Unit) {
+    when (state) {
+        is LoadState.Loading -> CircularProgressIndicator(Modifier.padding(16.dp))
+        is LoadState.Error ->
+            Text(
+                state.message,
+                color = MaterialTheme.colorScheme.error,
+                style = MaterialTheme.typography.bodySmall,
+                modifier = Modifier.padding(16.dp),
+            )
+        is LoadState.Loaded -> {
+            val listing = state.value
+            val sorted = remember(listing) { sortForDisplay(listing.entries) }
+            LazyColumn(Modifier.weight(1f).fillMaxWidth()) {
+                parentOf(listing.path)?.let { parent ->
+                    item("..") {
+                        EntryRow(
+                            glyph = FOLDER_GLYPH,
+                            name = "..",
+                            trailing = null,
+                            onClick = { onOpen(Spot.Dir(parent)) },
+                        )
+                    }
+                }
+                if (sorted.isEmpty()) {
+                    item("empty") {
+                        Text(
+                            "Nothing here",
+                            style = MaterialTheme.typography.bodySmall,
+                            color = MaterialTheme.colorScheme.onSurfaceVariant,
+                            modifier = Modifier.padding(16.dp),
+                        )
+                    }
+                }
+                uniqueItems(sorted, key = { it.name }) { entry ->
+                    val path = join(listing.path, entry.name)
+                    EntryRow(
+                        glyph = if (entry.isDirectory) FOLDER_GLYPH else FILE_GLYPH,
+                        name = entry.name,
+                        trailing = trailingOf(entry),
+                        onClick = {
+                            onOpen(if (entry.isDirectory) Spot.Dir(path) else Spot.Doc(path))
+                        },
+                    )
+                }
+            }
+        }
+    }
+}
+
+/**
+ * What a row says after the name, or nothing.
+ *
+ * A symlink says so instead of giving a size, because the size a listing reports for one is the
+ * length of the path it points at -- a number that looks exactly like a file size and is about
+ * something else entirely. `other` covers a fifo, a device, and a link whose target is gone: the
+ * row still appears, because a directory that hid what it held would be lying about being empty,
+ * and the word is there because a colour cannot say "this is a different kind of thing".
+ */
+private fun trailingOf(entry: DirEntry): String? =
+    when {
+        entry.link -> "link"
+        entry.isDirectory -> null
+        entry.kind == "file" -> humanSize(entry.size) ?: "0 B"
+        else -> "other"
+    }
+
+@Composable
+private fun EntryRow(glyph: String, name: String, trailing: String?, onClick: () -> Unit) {
+    Row(
+        verticalAlignment = Alignment.CenterVertically,
+        modifier =
+            Modifier.fillMaxWidth()
+                .clickable(onClick = onClick)
+                .padding(horizontal = 16.dp, vertical = 10.dp),
+    ) {
+        Glyph(glyph, colour = MaterialTheme.colorScheme.onSurfaceVariant)
+        Spacer(Modifier.width(12.dp))
+        Text(
+            name,
+            style = MaterialTheme.typography.bodyMedium,
+            maxLines = 1,
+            overflow = TextOverflow.MiddleEllipsis,
+            modifier = Modifier.weight(1f),
+        )
+        trailing?.let {
+            Spacer(Modifier.width(8.dp))
+            Text(
+                it,
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+        }
+    }
+}
+
+/**
+ * One file: read, and edited behind the pencil.
+ *
+ * Its own composable so that everything about one file -- what came back, what has been typed, and
+ * whether a save is out -- is remembered under that file's path and thrown away when the reader
+ * moves to another. What is *not* here is edit mode itself: back has to know about it, and back
+ * belongs to the screen.
+ */
+@Composable
+private fun ColumnScope.DocPane(
+    settings: ServerSettings,
+    target: FilesTarget,
+    path: String,
+    name: String,
+    editing: Boolean,
+    onEditing: (Boolean) -> Unit,
+    onDirty: (Boolean) -> Unit,
+    onBack: () -> Unit,
+) {
+    val scope = rememberCoroutineScope()
+    var state by remember(path) { mutableStateOf<LoadState<FileContent>>(LoadState.Loading) }
+    var draft by remember(path) { mutableStateOf(TextFieldValue()) }
+    var saving by remember(path) { mutableStateOf(false) }
+    var saveError by remember(path) { mutableStateOf<String?>(null) }
+    var conflict by remember(path) { mutableStateOf<String?>(null) }
+    // The editor's own vertical scroll, hoisted so the gutter and the text move together: they are
+    // two composables in one row, and a scroll inside either would leave the other behind.
+    val editScroll = rememberScrollState()
+    val language = remember(name) { fileLanguage(name) }
+    val loaded = (state as? LoadState.Loaded)?.value as? FileContent.Text
+
+    suspend fun fetch() {
+        state = LoadState.Loading
+        state =
+            try {
+                val got = withContext(Dispatchers.IO) { fetchFile(settings, target.setup, path) }
+                if (got is FileContent.Text) draft = TextFieldValue(got.content)
+                LoadState.Loaded(got)
+            } catch (e: ApiException) {
+                LoadState.failed(e)
+            }
+        onDirty(false)
+    }
+
+    LaunchedEffect(path) { fetch() }
+
+    val changed = loaded != null && draft.text != loaded.content
+    LaunchedEffect(changed) { onDirty(changed) }
+
+    /** Writes the draft back, [against] being the digest it is allowed to replace. */
+    fun save(against: String) {
+        if (saving) return
+        saving = true
+        saveError = null
+        scope.launch {
+            try {
+                val written =
+                    withContext(Dispatchers.IO) {
+                        writeFile(settings, target.setup, path, draft.text, against)
+                    }
+                state =
+                    LoadState.Loaded(
+                        FileContent.Text(
+                            path,
+                            written.size,
+                            written.modified,
+                            written.sha256,
+                            draft.text,
+                        )
+                    )
+                conflict = null
+                onDirty(false)
+                onEditing(false)
+            } catch (e: ApiException) {
+                // The one refusal that is a question rather than a message: somebody else's edit
+                // is on the machine, and which of the two survives is not this app's to decide.
+                if (e.status == 409) conflict = e.message ?: "It changed on the machine."
+                else saveError = e.message
+            } finally {
+                saving = false
+            }
+        }
+    }
+
+    FilesHeader(title = name, path = path, machine = target.setupName, onBack = onBack) {
+        if (editing) {
+            if (saving) {
+                GlyphSpinner("Saving")
+            } else {
+                GlyphButton(
+                    SAVE_GLYPH,
+                    "Save",
+                    { loaded?.let { save(it.sha256) } },
+                    // Disabled rather than hidden while there is nothing to write: a button that
+                    // comes and goes makes its own absence the signal.
+                    enabled = changed,
+                )
+            }
+        } else {
+            GlyphButton(
+                REFRESH_GLYPH,
+                "Read this file again",
+                { scope.launch { fetch() } },
+                enabled = state !is LoadState.Loading,
+            )
+            GlyphButton(EDIT_GLYPH, "Edit", { onEditing(true) }, enabled = loaded != null)
+        }
+    }
+
+    saveError?.let {
+        Text(
+            it,
+            color = MaterialTheme.colorScheme.error,
+            style = MaterialTheme.typography.bodySmall,
+            modifier = Modifier.padding(horizontal = 16.dp, vertical = 4.dp),
+        )
+    }
+
+    Box(Modifier.weight(1f).fillMaxWidth().background(rawSurface).padding(horizontal = 8.dp)) {
+        when (val current = state) {
+            is LoadState.Loading -> CircularProgressIndicator(Modifier.padding(8.dp))
+            is LoadState.Error ->
+                Text(
+                    current.message,
+                    color = MaterialTheme.colorScheme.error,
+                    style = MaterialTheme.typography.bodySmall,
+                    modifier = Modifier.padding(8.dp),
+                )
+            is LoadState.Loaded ->
+                when (val file = current.value) {
+                    is FileContent.Text ->
+                        if (editing) {
+                            FileEditor(
+                                draft,
+                                { draft = it },
+                                language,
+                                Modifier.verticalScroll(editScroll),
+                            )
+                        } else {
+                            val lines =
+                                remember(file.content, language) {
+                                    FileLines.of(file.content, language)
+                                }
+                            FileViewer(lines)
+                        }
+                    // Said in words, with the measurement that makes it make sense. Neither of
+                    // these is an empty file and neither is an error, so neither may look like one.
+                    is FileContent.Binary ->
+                        Note(
+                            "This is not text (${humanSize(file.size) ?: "0 B"}), so there is nothing to show."
+                        )
+                    is FileContent.TooBig ->
+                        Note(
+                            "This file is ${humanSize(file.size)}, which is more than the server will " +
+                                "send. Nothing was read, so nothing here is a sample of it."
+                        )
+                }
+        }
+    }
+
+    conflict?.let { message ->
+        ConflictDialog(
+            message = message,
+            busy = saving,
+            onOverwrite = {
+                // Re-read only to learn what it hashes to *now*, which is the digest an overwrite
+                // has to be allowed against. The content is deliberately thrown away: overwriting
+                // is the choice to lose it.
+                scope.launch {
+                    val fresh =
+                        try {
+                            withContext(Dispatchers.IO) { fetchFile(settings, target.setup, path) }
+                        } catch (e: ApiException) {
+                            saveError = e.message
+                            conflict = null
+                            return@launch
+                        }
+                    if (fresh is FileContent.Text) save(fresh.sha256)
+                    else {
+                        saveError =
+                            "It is no longer a text file, so this app will not write over it."
+                        conflict = null
+                    }
+                }
+            },
+            onReload = {
+                conflict = null
+                scope.launch { fetch() }
+            },
+            onCancel = { conflict = null },
+        )
+    }
+}
+
+/** A sentence where the file's content would be, for the two states that have no content. */
+@Composable
+private fun Note(text: String) {
+    Text(
+        text,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        modifier = Modifier.padding(8.dp),
+    )
+}
+
+/**
+ * Naming one thing in the directory that is open.
+ *
+ * A name and a switch, not a name and a body: the editor is where content is typed, and a modal
+ * with a text area in it is a second editor to keep in step with the first. A created file opens
+ * straight into edit mode, because an empty file is not something to look at.
+ */
+@Composable
+private fun CreateDialog(
+    settings: ServerSettings,
+    setup: String,
+    directory: String,
+    onDismiss: () -> Unit,
+    onCreated: (String, Boolean) -> Unit,
+) {
+    val scope = rememberCoroutineScope()
+    var name by remember { mutableStateOf("") }
+    var isDirectory by remember { mutableStateOf(false) }
+    var busy by remember { mutableStateOf(false) }
+    var error by remember { mutableStateOf<String?>(null) }
+
+    fun create() {
+        val chosen = name.trim()
+        if (busy || chosen.isEmpty()) return
+        busy = true
+        error = null
+        val path = join(directory, chosen)
+        scope.launch {
+            try {
+                withContext(Dispatchers.IO) {
+                    if (isDirectory) createDir(settings, setup, path)
+                    else createFile(settings, setup, path)
+                }
+                onCreated(path, isDirectory)
+            } catch (e: ApiException) {
+                // Beside the button that caused it: this dialog is the only thing on screen that
+                // knows something was being created, and the reason is usually the name itself.
+                error = e.message
+                busy = false
+            }
+        }
+    }
+
+    AlertDialog(
+        onDismissRequest = onDismiss,
+        title = { Text("Create in ${baseName(directory)}") },
+        text = {
+            Column {
+                OutlinedTextField(
+                    value = name,
+                    onValueChange = { name = it },
+                    label = { Text("Name") },
+                    singleLine = true,
+                    enabled = !busy,
+                    modifier = Modifier.fillMaxWidth(),
+                )
+                Spacer(Modifier.height(8.dp))
+                Row(verticalAlignment = Alignment.CenterVertically) {
+                    Text("Directory", modifier = Modifier.weight(1f))
+                    Switch(
+                        checked = isDirectory,
+                        onCheckedChange = { isDirectory = it },
+                        enabled = !busy,
+                    )
+                }
+                Text(
+                    "A name that is already taken is refused rather than replaced.",
+                    style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+                error?.let {
+                    Spacer(Modifier.height(8.dp))
+                    Text(
+                        it,
+                        color = MaterialTheme.colorScheme.error,
+                        style = MaterialTheme.typography.bodySmall,
+                    )
+                }
+            }
+        },
+        confirmButton = {
+            TextButton(onClick = { create() }, enabled = !busy && name.isNotBlank()) {
+                Text(if (busy) "Creating..." else "Create")
+            }
+        },
+        dismissButton = { TextButton(onClick = onDismiss, enabled = !busy) { Text("Cancel") } },
+    )
+}
+
+/**
+ * Directories first, then by name ignoring case, and stably.
+ *
+ * Sorted here rather than by the machine: presentation order is a display decision, and `find`
+ * answers in whatever order the directory happens to be stored in. Dotfiles are not hidden -- in a
+ * repository they are half of what matters.
+ */
+internal fun sortForDisplay(entries: List<DirEntry>): List<DirEntry> =
+    entries.sortedWith(compareBy({ !it.isDirectory }, { it.name.lowercase() }))
+
+/**
+ * The directory above [path], or null at the root.
+ *
+ * A string operation on a path the *machine* resolved, which is what makes it safe: every listing
+ * answers with its own `pwd -P`, so there is never a `..` or a symlink left in here to reason
+ * about, and this app never has to resolve one.
+ */
+internal fun parentOf(path: String): String? {
+    val trimmed = path.trimEnd('/')
+    if (trimmed.isEmpty()) return null
+    val cut = trimmed.lastIndexOf('/')
+    return when {
+        cut < 0 -> null
+        cut == 0 -> "/"
+        else -> trimmed.substring(0, cut)
+    }
+}
+
+/** What a path names: its last segment, with `/` naming itself. */
+internal fun baseName(path: String): String {
+    val trimmed = path.trimEnd('/')
+    return if (trimmed.isEmpty()) "/" else trimmed.substringAfterLast('/')
+}
+
+/** A resolved directory and a name in it, as one path. */
+internal fun join(directory: String, name: String): String =
+    if (directory.endsWith("/")) "$directory$name" else "$directory/$name"

app/androidApp/src/main/kotlin/com/example/aiapp/ImportScreen.kt

@@ -667,15 +667,6 @@ private fun ImportableList(
     }
 }
 
-/** A byte count at the coarsest unit that still says something, so rows stay comparable. */
-private fun humanSize(bytes: Long): String? =
-    when {
-        bytes <= 0L -> null
-        bytes >= 1_000_000L -> "${bytes / 1_000_000L} MB"
-        bytes >= 1_000L -> "${bytes / 1_000L} kB"
-        else -> "$bytes B"
-    }
-
 /** What this session is: the measurements, in the order they are worth knowing. */
 private fun statsOf(session: Importable): String =
     listOfNotNull(

app/androidApp/src/main/kotlin/com/example/aiapp/NerdIcons.kt

@@ -29,10 +29,10 @@ import androidx.compose.ui.unit.sp
  * grounds that a system font may not have the glyph and whoever gets the empty box instead is never
  * the person who wrote it. That objection is about *relying* on a system font, and it is exactly
  * right: the answer is not to avoid glyphs but to ship them. The font here is
- * `app/build-icon-font.sh`'s output -- eleven glyphs, 2.1 KB, subset out of the 3 MB symbols font
- * and committed -- so the codepoints below are resolved by an asset in the APK and cannot come back
- * as tofu. Adding one means adding its codepoint in *both* places; a codepoint here that the script
- * did not subset is a glyph that silently isn't there.
+ * `app/build-icon-font.sh`'s output -- seventeen glyphs, 2.8 KB, subset out of the 3 MB symbols
+ * font and committed -- so the codepoints below are resolved by an asset in the APK and cannot come
+ * back as tofu. Adding one means adding its codepoint in *both* places; a codepoint here that the
+ * script did not subset is a glyph that silently isn't there.
  *
  * The subset is the font's **Mono** face, where every glyph is exactly one em wide and one em tall.
  * That is what makes two icons the same size without either of them being given a size: the
@@ -117,6 +117,33 @@ val USAGE_GLYPH = glyph(0xF201)
  */
 val SPEED_GLYPH = glyph(0xF04C5)
 
+/**
+ * `md-folder` -- the files on the machine this session runs on.
+ *
+ * The same codepoint dev-updater uses, and it must not drift from it, for the reason the cog and
+ * the refresh arrow must not: a folder that meant something else in one of the two apps is exactly
+ * the confusion sharing them prevents. Doubles as the mark on a directory row inside the explorer,
+ * which is what makes the button say where it leads.
+ */
+val FOLDER_GLYPH = glyph(0xF024B)
+
+/** `md-file_outline` -- one file, in a listing beside the directories. */
+val FILE_GLYPH = glyph(0xF0224)
+
+/** `md-plus` -- make something here. dev-updater's codepoint as well. */
+val PLUS_GLYPH = glyph(0xF0415)
+
+/** `md-pencil` -- change what this file says, rather than only reading it. */
+val EDIT_GLYPH = glyph(0xF03EB)
+
+/**
+ * `md-content_save` -- write the edits back to the machine.
+ *
+ * The floppy disk, which is what save has meant for longer than most of the people reading it have
+ * been alive and is still the only mark anybody recognises for it.
+ */
+val SAVE_GLYPH = glyph(0xF0193)
+
 /**
  * The size an icon draws at beside a line of text.
  *

app/androidApp/src/main/kotlin/com/example/aiapp/SessionScreen.kt

@@ -236,6 +236,8 @@ fun SessionScreen(
     settings: ServerSettings,
     summary: SessionSummary,
     onBack: () -> Unit,
+    /** Opens the file explorer on this session's machine, starting where this session works. */
+    onFiles: (FilesTarget) -> Unit,
     /** What another app shared in while this session is the one open; see [ShareRequest]. */
     share: ShareRequest? = null,
     /** Said once [share] has been attached here, so it is not attached again. */
@@ -1225,6 +1227,60 @@ fun SessionScreen(
     // interpolated, so there is nothing for a dropped frame to interrupt), so it is what both
     // places below fall back to.
     val imeVisible = WindowInsets.isImeVisible
+
+    // What this session is costing to draw, copied out to somewhere it can be read.
+    //
+    // Written here rather than beside the control that runs it, because everything it measures --
+    // the events, the rows, the units, what the list has on screen, which cards are open -- is this
+    // composable's own state, and a control in a dialog cannot reach it. The control is a row in
+    // [SessionSettingsDialog]: that is where the session's other about-the-session controls are,
+    // and the header is for what a reader presses while reading. It copies rather than opens,
+    // because what it produces is for somewhere else -- a message to whoever is looking at the
+    // code -- and a screenful of timings read on the phone is a screenful nobody can act on.
+    //
+    // Whatever presses this, it is found by its **name**: `ui-trace`'s tap-by-label action resolves
+    // "Session settings" and then "Copy render timings" from what is on screen at that moment, so
+    // `transcript-bench.sh` and `stream-bench.sh` keep working when this moves again. They pressed
+    // it at a coordinate measured once by hand until 2026-09-03, and anything that moved the header
+    // made that tap land on whatever now sat there -- reporting a number that was never measured.
+    val copyRenderReport = {
+        val report =
+            debugReport(
+                device =
+                    "device: ${Build.MODEL} (${Build.MANUFACTURER})," +
+                        " Android ${Build.VERSION.RELEASE}\n" +
+                        // A debuggable build runs Compose at a fraction of release speed, so a
+                        // report that did not say which it came from was read as the app's own
+                        // cost.
+                        "build: ${if (debuggable(context)) "debug" else "release"}",
+                transcript =
+                    listOf(
+                        "  ${items.size} events, ${rows.size} rows, ${units.size} units loaded",
+                        "  viewport ${listState.layoutInfo.viewportSize.height}px," +
+                            " ${listState.layoutInfo.visibleItemsInfo.size} units visible",
+                        visibleUnits(units, listState.layoutInfo.visibleItemsInfo, UNITS_START),
+                        "  ${expandedTools.size} tool calls and ${expandedGroups.size} groups open",
+                    ),
+                frames = FrameStats.lines(context.refreshHz()),
+                accounting =
+                    FrameStats.drawPhase().let { (nanos, count) -> drawAccounting(nanos, count) },
+                crash = lastCrash(context),
+            )
+        context.copyToClipboard("ai-app render report", report)
+        // Also to the log, so a session driving the app over adb can read the same report the
+        // button copies. The clipboard is not reachable from a shell, and a counter nobody can
+        // check from here is a counter that only gets checked by asking Iris to press a button
+        // and paste.
+        Log.i("ai-app", report)
+        // Only once it is somewhere it can be read from, so a copy that never happened does not
+        // throw the stack away with it.
+        clearCrash(context)
+        // Emptied by the copy, so pressing it twice measures two separate stretches of scrolling
+        // rather than one and then the same one again.
+        FrameStats.reset()
+        DebugStats.reset()
+        Toast.makeText(context, "Copied render report", Toast.LENGTH_SHORT).show()
+    }
     Box(Modifier.fillMaxSize()) {
         Column(Modifier.fillMaxSize()) {
             Row(
@@ -1266,74 +1322,36 @@ fun SessionScreen(
                 // yellow or red near a limit -- and the theme's plain control colour whenever there
                 // is no measurement, since blue is the low end of the scale here and would read as
                 // "checked, and fine" about a machine nobody could reach.
+                // Usage, files, settings -- widest scope first, narrowing to the right, so the
+                // cog stays at the end where every other screen keeps it. Asked for in this order
+                // by Iris on 2026-09-03.
                 Row {
-                    // Left of the numbers about the *conversation*, because it is the same kind of
-                    // thing about the *app*: what this session is costing to draw. It copies rather
-                    // than opens, because what it produces is for somewhere else -- a message to
-                    // whoever is looking at the code -- and a screenful of timings read on the
-                    // phone
-                    // is a screenful nobody can act on.
-                    GlyphButton(
-                        SPEED_GLYPH,
-                        "Copy render timings",
-                        onClick = {
-                            val report =
-                                debugReport(
-                                    device =
-                                        "device: ${Build.MODEL} (${Build.MANUFACTURER})," +
-                                            " Android ${Build.VERSION.RELEASE}\n" +
-                                            // A debuggable build runs Compose at a fraction of
-                                            // release speed, so a report that did not say which
-                                            // it came from was read as the app's own cost.
-                                            "build: ${if (debuggable(context)) "debug" else "release"}",
-                                    transcript =
-                                        listOf(
-                                            "  ${items.size} events, ${rows.size} rows," +
-                                                " ${units.size} units loaded",
-                                            "  viewport" +
-                                                " ${listState.layoutInfo.viewportSize.height}px," +
-                                                " ${listState.layoutInfo.visibleItemsInfo.size}" +
-                                                " units visible",
-                                            visibleUnits(
-                                                units,
-                                                listState.layoutInfo.visibleItemsInfo,
-                                                UNITS_START,
-                                            ),
-                                            "  ${expandedTools.size} tool calls and" +
-                                                " ${expandedGroups.size} groups open",
-                                        ),
-                                    frames = FrameStats.lines(context.refreshHz()),
-                                    accounting =
-                                        FrameStats.drawPhase().let { (nanos, count) ->
-                                            drawAccounting(nanos, count)
-                                        },
-                                    crash = lastCrash(context),
-                                )
-                            context.copyToClipboard("ai-app render report", report)
-                            // Also to the log, so a session driving the app over adb can read the
-                            // same report the button copies. The clipboard is not reachable from a
-                            // shell, and a counter nobody can check from here is a counter that
-                            // only
-                            // gets checked by asking Iris to press a button and paste.
-                            Log.i("ai-app", report)
-                            // Only once it is somewhere it can be read from, so a copy that never
-                            // happened does not throw the stack away with it.
-                            clearCrash(context)
-                            // Emptied by the copy, so pressing it twice measures two separate
-                            // stretches
-                            // of scrolling rather than one and then the same one again.
-                            FrameStats.reset()
-                            DebugStats.reset()
-                            Toast.makeText(context, "Copied render report", Toast.LENGTH_SHORT)
-                                .show()
-                        },
-                    )
                     GlyphButton(
                         USAGE_GLYPH,
                         "Usage",
                         { usageOpen = true },
                         colour = usageGlyphColour(usage),
                     )
+                    // The machine's files, which is where the answer to "what did it actually
+                    // change" is. It opens *over* this screen rather than replacing it -- see
+                    // [Screen.Session].
+                    GlyphButton(
+                        FOLDER_GLYPH,
+                        "Files",
+                        onClick = {
+                            onFiles(
+                                FilesTarget(
+                                    setup = summary.setup,
+                                    setupName = summary.setupName,
+                                    // Where this session works, and the machine's own home when it
+                                    // was never given a directory -- resolved there rather than
+                                    // guessed at here, since this app does not know that machine's
+                                    // home and must not invent one.
+                                    start = summary.cwd?.takeIf { it.isNotBlank() } ?: "~",
+                                )
+                            )
+                        },
+                    )
                     // What it opens is about this session, so it sits at the end of the session's
                     // own row. The name is the whole of what it holds today, which is why it is a
                     // cog
@@ -1948,6 +1966,7 @@ fun SessionScreen(
                 settingsOpen = false
             },
             onDismiss = { settingsOpen = false },
+            onCopyRenderReport = copyRenderReport,
         )
     }
 }

app/androidApp/src/main/kotlin/com/example/aiapp/SessionSettingsDialog.kt

@@ -56,6 +56,11 @@ fun SessionSettingsDialog(
     title: String,
     onRenamed: (String) -> Unit,
     onDismiss: () -> Unit,
+    /**
+     * Copies what this session costs to draw. Built by the session screen, because everything it
+     * measures is that screen's own state -- see `copyRenderReport` there.
+     */
+    onCopyRenderReport: () -> Unit,
 ) {
     val scope = rememberCoroutineScope()
     var name by remember(sessionId) { mutableStateOf(title) }
@@ -258,6 +263,20 @@ fun SessionSettingsDialog(
                         style = MaterialTheme.typography.bodySmall,
                     )
                 }
+                Spacer(Modifier.height(8.dp))
+                // About this session, which is what everything in here is -- and it was on the
+                // header until 2026-09-03, where the folder button now is. It copies rather than
+                // opening anything, so it says so and then says it happened: a row that looks like
+                // a control and gives no sign of having run is one people press twice.
+                Row(
+                    verticalAlignment = Alignment.CenterVertically,
+                    modifier = Modifier.fillMaxWidth(),
+                ) {
+                    Glyph(SPEED_GLYPH, colour = MaterialTheme.colorScheme.onSurface)
+                    Spacer(Modifier.width(8.dp))
+                    Text("Render timings", modifier = Modifier.weight(1f))
+                    TextButton(onClick = onCopyRenderReport) { Text("Copy") }
+                }
             }
         },
         // Disabled rather than absent while there is nothing to save: a button that comes and

app/androidApp/src/main/kotlin/com/example/aiapp/Sizes.kt

@@ -0,0 +1,20 @@
+package com.example.aiapp
+
+/**
+ * A byte count at the coarsest unit that still says something, so rows stay comparable.
+ *
+ * Null at zero and below, because the two screens that ask disagree about what nothing means and
+ * only the caller knows: a transcript of no bytes is a measurement that has not happened, and is
+ * left off the row; a file of no bytes is a file with nothing in it, and the explorer says `0 B`
+ * rather than leaving a gap the reader would have to interpret.
+ *
+ * Its own file rather than the import screen's, where it started: two screens now say a size, and a
+ * second copy of these thresholds is how one list comes to call 4 kB what the other calls 4096 B.
+ */
+fun humanSize(bytes: Long): String? =
+    when {
+        bytes <= 0L -> null
+        bytes >= 1_000_000L -> "${bytes / 1_000_000L} MB"
+        bytes >= 1_000L -> "${bytes / 1_000L} kB"
+        else -> "$bytes B"
+    }

app/androidApp/src/main/kotlin/com/example/aiapp/Sse.kt

@@ -93,7 +93,7 @@ class Sse(private val settings: ServerSettings) {
             if (!closed) {
                 throw ApiException(
                     "Can't reach the server -- retrying. (${e.message ?: e::class.simpleName})",
-                    e,
+                    cause = e,
                 )
             }
         } finally {

app/androidApp/src/test/kotlin/com/example/aiapp/FileLinesTest.kt

@@ -0,0 +1,73 @@
+package com.example.aiapp
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+/**
+ * The line arithmetic behind the file viewer.
+ *
+ * Worth a test rather than an eye: a line number that is one out is invisible in a short file and
+ * obvious in a long one, and a colour that stops at a line break is invisible until the file has a
+ * block comment in it.
+ */
+class FileLinesTest {
+    @Test
+    fun `a file that ends with a newline has the number of lines its author would count`() {
+        assertEquals(listOf("one", "two"), FileLines.of("one\ntwo\n", null).lines)
+        assertEquals(listOf("one", "two"), FileLines.of("one\ntwo", null).lines)
+        // Only one is dropped: a blank line at the end of a file is a line somebody typed.
+        assertEquals(listOf("one", "two", ""), FileLines.of("one\ntwo\n\n", null).lines)
+    }
+
+    @Test
+    fun `an empty file is one empty line`() {
+        val lines = FileLines.of("", null)
+        assertEquals(1, lines.size)
+        assertEquals("", lines.line(0).text)
+    }
+
+    @Test
+    fun `a comment that spans lines is coloured on every line it covers`() {
+        val text = "fn a() {}\n/* still\n   a comment */\nfn b() {}\n"
+        val lines = FileLines.of(text, Language.RUST)
+        assertEquals(4, lines.size)
+        val comment = catppuccinSyntax().of(Kind.COMMENT)
+        // The whole of the middle line, and the part of the third up to the closer.
+        assertTrue(lines.line(1).spanStyles.any { it.item.color == comment && it.start == 0 })
+        val third = lines.line(2)
+        assertTrue(third.spanStyles.any { it.item.color == comment && it.end == third.length })
+        // And the code around it is not commented.
+        assertTrue(lines.line(0).spanStyles.none { it.item.color == comment })
+        assertTrue(lines.line(3).spanStyles.none { it.item.color == comment })
+    }
+
+    @Test
+    fun `a span never runs past the line it was cut into`() {
+        val lines = FileLines.of("val x = \"a\nb\"\nval y = 1\n", Language.KOTLIN)
+        for (index in 0 until lines.size) {
+            val line = lines.line(index)
+            assertTrue(
+                line.spanStyles.all { it.start >= 0 && it.end <= line.length },
+                "line $index",
+            )
+        }
+    }
+
+    @Test
+    fun `a file with no language is plain`() {
+        val lines = FileLines.of("fn main() {}\n", null)
+        assertTrue(lines.line(0).spanStyles.isEmpty())
+    }
+
+    @Test
+    fun `a language comes from the extension, and only from a real one`() {
+        assertEquals(Language.KOTLIN, fileLanguage("Main.kt"))
+        assertEquals(Language.KOTLIN, fileLanguage("build.gradle.kts"))
+        assertEquals(Language.RUST, fileLanguage("files.rs"))
+        assertEquals(Language.TOML, fileLanguage("Cargo.toml"))
+        assertEquals(null, fileLanguage("Makefile"))
+        assertEquals(null, fileLanguage(".bashrc"))
+        assertEquals(null, fileLanguage("notes.txt"))
+    }
+}

app/bench-lib.sh

@@ -0,0 +1,57 @@
+# Shared by transcript-bench.sh and stream-bench.sh: the parts of driving
+# this app that both need and that neither should describe twice.
+#
+# Sourced, not run. It assumes android-env.sh has already been sourced, so
+# `ui-trace` and `adb` are this checkout's.
+#
+# **Nothing here presses a coordinate.** Every control is found by the name
+# it already carries for assistive technology, resolved from the screen at
+# the moment of the gesture (`ui-trace record --do "tap 'Save'"`). A
+# coordinate is a position measured once by hand, and anything that moves
+# the control -- a button added to the row, a font size, another device --
+# makes the tap land on whatever now sits there; the bench then reports a
+# number that was never measured, which reads exactly like a result. That
+# is not hypothetical: both scripts pressed the render-report button at
+# `tap 723 205` until 2026-09-03, when it moved into the session settings
+# dialog. The check that none has crept back:
+#
+#     grep -n "tap [0-9]" app/*.sh
+
+# The title of the session these benches open by default: whichever one the
+# server lists first, which is the most recently active.
+#
+# Asked of the server rather than taken from the screen, because a row has
+# no stable name of its own and "the first one" is a fact about the list
+# rather than about a position on it. Passing -s overrides it, and that is
+# the honest way to measure a particular session: the list is ordered by
+# last activity, so "the first row" is a different session at different
+# times of day.
+first_session_title() {
+    ./ui-sandbox.sh api /sessions |
+        python3 -c 'import json,sys; d=json.load(sys.stdin); print(d[0]["title"] if d else "")'
+}
+
+# Restarts the app and opens the session called $1.
+open_session() {
+    adb shell am force-stop com.example.aiapp
+    adb shell am start -n com.example.aiapp/.MainActivity >/dev/null
+    sleep 5
+    ui-trace record -d 6000 --do "tap '$1'" --do 'wait 3000' \
+        -o /tmp/bench-open.txt >/dev/null
+    sleep 2
+}
+
+# Copies the app's render report to the clipboard and the log, through the
+# session settings dialog it lives in.
+#
+# Pressed twice per run: once to empty the counters, and once at the end,
+# so what comes back covers exactly the work between the two. Closing the
+# dialog is part of it -- it covers the transcript, which is the thing the
+# gestures in between are meant to reach.
+copy_render_report() {
+    ui-trace record -d 7000 \
+        --do "tap 'Session settings'" --do 'wait 1500' \
+        --do "tap 'Copy'" --do 'wait 800' \
+        --do "tap 'Close'" --do 'wait 800' \
+        -o "${1:-/tmp/bench-report.txt}" >/dev/null
+}

app/build-icon-font.sh

@@ -39,6 +39,11 @@ GLYPHS=(
     U+F004D  # md-arrow_left
     U+F009A  # md-bell
     U+F04C5  # md-speedometer
+    U+F024B  # md-folder -- dev-updater's too; a folder means the same in both
+    U+F0415  # md-plus -- likewise
+    U+F03EB  # md-pencil
+    U+F0193  # md-content_save
+    U+F0224  # md-file_outline
     U+F201   # fa-line_chart -- Font Awesome's, asked for by name
 )
 

app/stream-bench.sh

@@ -4,8 +4,9 @@
 # reply that is still arriving cheap", the way transcript-bench.sh is for
 # scrolling one that has settled.
 #
-#   ./stream-bench.sh FILE           restart the app, open the first
-#                                    session, stream FILE into it, report
+#   ./stream-bench.sh FILE           restart the app, open the session the
+#                                    sandbox lists first, stream FILE into
+#                                    it, report
 #   ./stream-bench.sh -k FILE        keep whatever session is open now
 #
 # The session is the first one the sandbox lists (ui-sandbox.sh spawn makes
@@ -17,6 +18,7 @@
 set -eu
 cd "$(dirname "$0")"
 . ./android-env.sh >/dev/null 2>&1
+. ./bench-lib.sh
 
 keep=""
 while getopts k flag; do
@@ -31,11 +33,9 @@ file=${1:?usage: stream-bench.sh [-k] FILE}
 sid=$(./ui-sandbox.sh api /sessions | python3 -c 'import json,sys; print(json.load(sys.stdin)[0]["id"])')
 
 if [ -z "$keep" ]; then
-    adb shell am force-stop com.example.aiapp
-    adb shell am start -n com.example.aiapp/.MainActivity >/dev/null
-    sleep 5
-    ui-trace record -d 3000 --do 'tap 500 545' -o /tmp/bench-open.txt >/dev/null 2>&1
-    sleep 3
+    title=$(first_session_title)
+    [ -n "$title" ] || { echo "stream-bench: no sessions to open" >&2; exit 1; }
+    open_session "$title"
 fi
 
 # Pinned to the newest end before anything is sent. The transcript never
@@ -46,18 +46,17 @@ fi
 # reads exactly like a build where the work vanished. The control is the
 # app's own "Jump to latest", which is only there while the newest message
 # is off screen -- so when it is absent the list is already where it needs
-# to be.
-jump=$(ui-trace record -d 1200 -o /tmp/bench-jump.txt >/dev/null 2>&1
-    ui-trace show /tmp/bench-jump.txt -m 'Jump to latest' --field box |
-        grep -o '[0-9]*,[0-9]*\.\.[0-9]*,[0-9]*' | tail -1)
-if [ -n "$jump" ]; then
-    x=$(echo "$jump" | awk -F'[,.]' '{print int(($1 + $4) / 2)}')
-    y=$(echo "$jump" | awk -F'[,.]' '{print int(($2 + $5) / 2)}')
-    ui-trace record -d 1500 --do "tap $x $y" -o /tmp/bench-tolatest.txt >/dev/null 2>&1
+# to be. Looked for before it is pressed rather than pressed blindly: a tap
+# by name fails the whole run when its label is missing, and missing is the
+# ordinary case here.
+ui-trace record -d 1200 -o /tmp/bench-jump.txt >/dev/null 2>&1
+if ui-trace show /tmp/bench-jump.txt -m 'Jump to latest' --field box | grep -q '[0-9],[0-9]'; then
+    ui-trace record -d 2500 --do "tap 'Jump to latest'" --do 'wait 1000' \
+        -o /tmp/bench-tolatest.txt >/dev/null
 fi
 
-# The first tap resets the report's window; see transcript-bench.sh.
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-reset.txt >/dev/null 2>&1
+# The first copy resets the report's window; see transcript-bench.sh.
+copy_render_report /tmp/bench-reset.txt
 adb logcat -c
 
 ./ui-sandbox.sh send "$sid" "@$file" >/dev/null
@@ -90,6 +89,6 @@ while [ "$i" -lt 180 ]; do
 done
 sleep 1
 
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-report.txt >/dev/null 2>&1
+copy_render_report /tmp/bench-report.txt
 sleep 1
 adb logcat -d -s ai-app:I | sed -n '/ai-app render report/,$p' | sed 's/^.*ai-app  : //'

app/transcript-bench.sh

@@ -3,8 +3,9 @@
 # standard measurement for "is the transcript smooth", so a perf session
 # starts from a number instead of re-inventing this loop in /tmp.
 #
-#   ./transcript-bench.sh            restart the app, open the first
-#                                    session, scroll, report
+#   ./transcript-bench.sh            restart the app, open the session the
+#                                    server lists first, scroll, report
+#   ./transcript-bench.sh -s TITLE   open that session instead
 #   ./transcript-bench.sh -k         keep whatever screen is open now
 #   ./transcript-bench.sh -n 4       swipe cycles (default 6; one cycle is
 #                                    two swipes back and two forward)
@@ -16,33 +17,35 @@
 # went. Compare two runs of this with the same gestures, not one run
 # against a phone.
 #
-# Coordinates are the ai-app AVD's 1080x2400 screen: the report button in
-# the session top bar, and the first row of the session list.
+# The swipes are still coordinates, because a gesture across a scrolling
+# area is a distance rather than a control. Every *control* is pressed by
+# name -- see bench-lib.sh.
 set -eu
 cd "$(dirname "$0")"
 . ./android-env.sh >/dev/null 2>&1
+. ./bench-lib.sh
 
 keep=""
 cycles=6
-while getopts kn: flag; do
+session=""
+while getopts kn:s: flag; do
     case $flag in
     k) keep=1 ;;
     n) cycles=$OPTARG ;;
+    s) session=$OPTARG ;;
     *) exit 2 ;;
     esac
 done
 
 if [ -z "$keep" ]; then
-    adb shell am force-stop com.example.aiapp
-    adb shell am start -n com.example.aiapp/.MainActivity >/dev/null
-    sleep 5
-    ui-trace record -d 3000 --do 'tap 500 545' -o /tmp/bench-open.txt >/dev/null 2>&1
-    sleep 3
+    [ -n "$session" ] || session=$(first_session_title)
+    [ -n "$session" ] || { echo "transcript-bench: no sessions to open" >&2; exit 1; }
+    open_session "$session"
 fi
 
-# First tap resets the report's "work since last copied" window, so the
-# numbers cover exactly the swipes between the two taps.
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-reset.txt >/dev/null 2>&1
+# The first copy resets the report's "work since last copied" window, so the
+# numbers cover exactly the swipes between the two.
+copy_render_report /tmp/bench-reset.txt
 adb logcat -c
 
 DO=""
@@ -56,7 +59,7 @@ while [ "$i" -lt "$cycles" ]; do
 done
 eval ui-trace record -d $((cycles * 4400 + 2000)) $DO -o /tmp/bench-scroll.txt >/dev/null 2>&1
 
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-report.txt >/dev/null 2>&1
+copy_render_report /tmp/bench-report.txt
 sleep 1
 adb logcat -d -s ai-app:I | sed -n '/ai-app render report/,$p' | sed 's/^.*ai-app  : //'
 adb logcat -d 2>/dev/null | grep -i "duplicate unit key" | head -3 || true