Restore directory navigation on Android back

EXPLORER.md

@@ -208,17 +208,18 @@ absence the signal. Back with unsaved changes asks, and says the edits will
 be lost. The explorer draws over the session, which deliberately has no
 `imePadding`, so the explorer's own box adds it.
 
-### 10. The explorer draws over the session, and back closes it first
+### 10. The explorer draws over the session, and the two back controls differ
 
 `Screen.Session` in `AppRoot` gains a `files: FilesTarget?`. When set, the
 `FilesScreen` is composed **on top of** the session in the same `Box`, and
 the session stays composed under it: its event stream keeps flowing, its
 scroll position and draft stay where they were, and returning from a file
-costs nothing. Back — the button and the platform gesture — returns from an
-open file to its containing directory, then clears `files` and returns to the
-session. Directory navigation stays in the listing: `..` is an explicit row
-rather than a hidden second meaning for Back. An editor with unsaved changes
-asks before returning to the directory. "Back returns; it does not exit."
+costs nothing. The header's back button clears `files` and returns directly to
+the session. Android back returns from an open file to its containing directory,
+then walks to the parent directory until the root; at the root it returns to the
+session. The `..` row remains as the visible, tappable form of the same directory
+movement. An editor with unsaved changes asks before either route discards them.
+"Back returns; it does not exit."
 
 Rejected: a `Screen.Files` beside `Screen.Session`. Every route back from a
 leaf screen goes to Main today, and a session disposed and re-created on each

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

@@ -72,13 +72,18 @@ private sealed class Spot(val path: String) {
     class Doc(path: String, val directory: Dir) : Spot(path)
 }
 
+private enum class UnsavedDestination {
+    Directory,
+    Session,
+}
+
 /**
  * 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 and coming back from a file costs nothing. Back from a file returns to its directory;
- * back from a directory closes the explorer and returns to the session. Directory navigation stays
- * inside the listing, where its `..` row is explicit.
+ * flowing and coming back from a file costs nothing. Android back returns from a file to its
+ * directory, then walks up the directory tree. The header's back button closes the explorer and
+ * returns directly to the session.
  *
  * Every directory that has been visited is kept for as long as this is open; the refresh glyph is
  * how one gets asked again on purpose, and creating something refetches the directory it was
@@ -98,11 +103,10 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
     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.
+    // because both ways out have to ask before discarding it.
     var editing by remember { mutableStateOf(false) }
     var dirty by remember { mutableStateOf(false) }
-    var askUnsaved by remember { mutableStateOf(false) }
+    var unsavedDestination by remember { mutableStateOf<UnsavedDestination?>(null) }
 
     fun go(spot: Spot) {
         editing = false
@@ -110,11 +114,23 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
         here = spot
     }
 
-    fun back() {
-        when {
-            editing && dirty -> askUnsaved = true
-            here is Spot.Doc -> go((here as Spot.Doc).directory)
-            else -> onClose()
+    fun leave(destination: UnsavedDestination) {
+        if (editing && dirty) {
+            unsavedDestination = destination
+        } else if (destination == UnsavedDestination.Directory) {
+            go((here as Spot.Doc).directory)
+        } else {
+            onClose()
+        }
+    }
+
+    fun systemBack() {
+        when (val spot = here) {
+            is Spot.Doc -> leave(UnsavedDestination.Directory)
+            is Spot.Dir -> {
+                val path = (listings[spot.path] as? LoadState.Loaded)?.value?.path ?: spot.path
+                parentOf(path)?.let { go(Spot.Dir(it)) } ?: onClose()
+            }
         }
     }
 
@@ -131,7 +147,7 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
             }
     }
 
-    BackHandler(onBack = ::back)
+    BackHandler(onBack = ::systemBack)
 
     Box(
         Modifier.fillMaxSize()
@@ -151,7 +167,7 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
                         title = baseName(at),
                         path = at,
                         machine = target.setupName,
-                        onBack = ::back,
+                        onBack = { leave(UnsavedDestination.Session) },
                     ) {
                         GlyphButton(
                             REFRESH_GLYPH,
@@ -178,20 +194,23 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
                         editing = editing,
                         onEditing = { editing = it },
                         onDirty = { dirty = it },
-                        onBack = ::back,
+                        onBack = { leave(UnsavedDestination.Session) },
                     )
             }
         }
     }
 
-    if (askUnsaved) {
+    unsavedDestination?.let { destination ->
         UnsavedDialog(
             onDiscard = {
-                askUnsaved = false
-                val spot = here
-                if (spot is Spot.Doc) go(spot.directory) else onClose()
+                unsavedDestination = null
+                if (destination == UnsavedDestination.Directory) {
+                    go((here as Spot.Doc).directory)
+                } else {
+                    onClose()
+                }
             },
-            onCancel = { askUnsaved = false },
+            onCancel = { unsavedDestination = null },
         )
     }