Keep the image on screen when its tool call joins a group

A `Read` that returns an image is a row of one call, and the moment the
session makes its next call the two become a group -- which is a different
composable in a different part of the tree, so the old subtree goes and
everything it remembered goes with it. The full-screen viewer was inside
that subtree, so somebody looking at a screenshot was thrown back to the
transcript because the session carried on working. A page of history
landing does the same thing to the same row.

What is open is a property of the screen rather than of whichever row
happened to draw the thumbnail, so it is held there now and drawn beside
the other two dialogs. Nothing that happens to rows can reach it.

The cost is one fetch when it opens, since the thumbnail's decoded bitmap
belongs to a row this no longer goes through. Paid deliberately rather than
plumbed around: it is one request for a picture somebody asked to see, and
the viewer draws the same two empty states the thumbnail does -- still
coming, and never coming -- which it previously could not have, since it
only ever opened on a bitmap already in hand.

`/tools n gap` now puts a screenshot on its first call, so the case is
reproducible rather than argued about: that command already existed to make
a run *grow* while somebody watches, and the image is what made growing
matter. Checked on the emulator with `/tools 3 30` -- opened the image on
the lone call, and it was still open a minute later with the row by then
inside a group of three, and back returned to the transcript rather than
leaving the app.

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

@@ -78,12 +78,18 @@ fun rememberSessionBitmap(settings: ServerSettings, sessionId: String, ref: Stri
  * is what it should be.
  *
  * Four lines of body text, so a screenshot reads as an attachment beside the conversation rather
- * than as a page of its own. Full size is one tap away.
+ * than as a page of its own. Full size is one tap away -- but the full-size view itself is not
+ * here. [onOpen] hands the ref to the screen, which draws [SessionImageViewer] outside the list;
+ * see that function for the reason.
  */
 @Composable
-fun SessionImage(settings: ServerSettings, sessionId: String, ref: String) {
+fun SessionImage(
+    settings: ServerSettings,
+    sessionId: String,
+    ref: String,
+    onOpen: (String) -> Unit,
+) {
     val (bitmap, failed) = rememberSessionBitmap(settings, sessionId, ref)
-    var full by remember(ref) { mutableStateOf(false) }
     val height = thumbnailHeight()
     val heightPx = with(LocalDensity.current) { height.roundToPx() }
     Box(Modifier.fillMaxWidth().height(height), contentAlignment = Alignment.CenterStart) {
@@ -102,12 +108,60 @@ fun SessionImage(settings: ServerSettings, sessionId: String, ref: String) {
                     contentDescription = "Attached image, tap to view full screen",
                     contentScale = ContentScale.Fit,
                     filterQuality = enlargingFilter(image.height, heightPx),
-                    modifier = Modifier.fillMaxSize().clickable { full = true },
+                    modifier = Modifier.fillMaxSize().clickable { onOpen(ref) },
                     alignment = Alignment.CenterStart,
                 )
         }
     }
-    if (full) bitmap?.let { image -> ImageViewer(image) { full = false } }
+}
+
+/**
+ * The image somebody opened, drawn by the screen rather than by the row it was tapped in.
+ *
+ * The row is the wrong place to hold this, and it took a real fault to see why: an image from a
+ * `Read` on its own is a row of one call, and the moment the next call arrives the two become a
+ * group -- a different composable in a different part of the tree, so everything the old subtree
+ * remembered goes, the dialog included. Somebody looking at a screenshot was thrown back to the
+ * transcript because the session made another tool call. The same happens to a row regrouped by a
+ * page of history landing.
+ *
+ * Held by the screen, none of that reaches it: what is open is a property of the screen, not of
+ * whichever row happened to draw the thumbnail.
+ *
+ * The cost is one fetch, since the thumbnail's decoded bitmap belongs to a row this does not go
+ * through. Paid deliberately rather than plumbed around: it is one request for a picture somebody
+ * asked to see, and the loading and unavailable states below are the same two the thumbnail draws.
+ */
+@Composable
+fun SessionImageViewer(
+    settings: ServerSettings,
+    sessionId: String,
+    ref: String,
+    onClose: () -> Unit,
+) {
+    val (bitmap, failed) = rememberSessionBitmap(settings, sessionId, ref)
+    Dialog(
+        onDismissRequest = onClose,
+        properties = DialogProperties(usePlatformDefaultWidth = false),
+    ) {
+        Box(
+            Modifier.fillMaxSize().background(Color.Black).clickable(onClick = onClose),
+            contentAlignment = Alignment.Center,
+        ) {
+            when (val image = bitmap) {
+                // Two states, not one, exactly as the thumbnail has them: still coming, and never
+                // coming. Stated in white because this box paints its own black behind them and a
+                // theme colour would be picked against a surface that is not there.
+                null ->
+                    Text(
+                        if (failed) "Image $ref is unavailable" else "Loading image…",
+                        color = Color.White,
+                        style = MaterialTheme.typography.bodyMedium,
+                    )
+                else -> ZoomableImage(image)
+            }
+        }
+    }
 }
 
 /**
@@ -139,23 +193,23 @@ private fun enlargingFilter(sourceHeight: Int, drawnHeight: Int): FilterQuality
 /**
  * The image on its own, as large as it fits, with pinch to zoom.
  *
- * A dialog rather than a screen, so the platform's back gesture returns to the transcript instead
- * of leaving the app. It opens fitted -- the whole image visible, which is the thing a reader wants
- * first -- and zoom is theirs from there.
+ * Inside a dialog rather than a screen -- see [SessionImageViewer] -- so the platform's back
+ * gesture returns to the transcript instead of leaving the app. It opens fitted, the whole image
+ * visible, which is the thing a reader wants first; zoom is theirs from there.
  */
 @Composable
-private fun ImageViewer(image: ImageBitmap, onClose: () -> Unit) {
-    Dialog(
-        onDismissRequest = onClose,
-        properties = DialogProperties(usePlatformDefaultWidth = false),
-    ) {
-        var scale by remember { mutableFloatStateOf(1f) }
-        var offsetX by remember { mutableFloatStateOf(0f) }
-        var offsetY by remember { mutableFloatStateOf(0f) }
-        Box(
+private fun ZoomableImage(image: ImageBitmap) {
+    var scale by remember { mutableFloatStateOf(1f) }
+    var offsetX by remember { mutableFloatStateOf(0f) }
+    var offsetY by remember { mutableFloatStateOf(0f) }
+    Image(
+        bitmap = image,
+        contentDescription = "Attached image",
+        contentScale = ContentScale.Fit,
+        // Zoomed in, the reader is looking at pixels on purpose.
+        filterQuality = FilterQuality.None,
+        modifier =
             Modifier.fillMaxSize()
-                .background(Color.Black)
-                .clickable(onClick = onClose)
                 .pointerInput(Unit) {
                     detectTransformGestures { _, pan, zoom, _ ->
                         // Floor of 1 so the image cannot be pinched smaller than fitted, which is
@@ -169,23 +223,12 @@ private fun ImageViewer(image: ImageBitmap, onClose: () -> Unit) {
                             offsetY = 0f
                         }
                     }
+                }
+                .graphicsLayer {
+                    scaleX = scale
+                    scaleY = scale
+                    translationX = offsetX
+                    translationY = offsetY
                 },
-            contentAlignment = Alignment.Center,
-        ) {
-            Image(
-                bitmap = image,
-                contentDescription = "Attached image",
-                contentScale = ContentScale.Fit,
-                // Zoomed in, the reader is looking at pixels on purpose.
-                filterQuality = FilterQuality.None,
-                modifier =
-                    Modifier.fillMaxSize().graphicsLayer {
-                        scaleX = scale
-                        scaleY = scale
-                        translationX = offsetX
-                        translationY = offsetY
-                    },
-            )
-        }
-    }
+    )
 }