Stop the composer sticking above the bottom of the screen after the keyboard closes

Reported: closing the keyboard on purpose, while a reply was streaming,
left the composer floating above the bottom of the screen for the rest of
the session -- a bar of background colour under it, nothing that closed it.

The composer's position and the transcript's bottom padding are both driven
by the raw, animated `WindowInsets.ime` value, read inside a `graphicsLayer`
block specifically so a keyboard frame invalidates layer properties only
rather than recomposing the whole screen (see the layout note above it).
That value is carried by a `WindowInsetsAnimationCallback`, and a callback
interrupted mid-flight leaves whatever it was carrying frozen at its last
value with nothing left to correct it -- no further keyboard movement is
coming to fire the callback again. A streaming reply invalidates the view
every frame, which is exactly the condition known to starve a running
callback of its `onEnd`, and that is the "actively responding sessions"
correlate in the report.

`WindowInsets.isImeVisible` doesn't share that failure mode: it is set once,
from the platform's own start/end of the transition, over a different path
(`onApplyWindowInsets` rather than the animation callback) -- so it cannot
get stuck mid-animation the way the interpolated value can. Read once per
keyboard toggle and used to force both the composer's translation and the
transcript's reserved padding back to exactly zero the moment the platform
says the keyboard is gone, whatever the animated value still claims.

Checked on the emulator with an actively streaming echo session: opened the
keyboard, closed it with the system back gesture while the reply kept
growing, and the composer settled flush at the bottom with the transcript
filling the freed space, both immediately and after the keyboard was
reopened and closed again.

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

@@ -13,6 +13,7 @@ import androidx.compose.foundation.gestures.awaitEachGesture
 import androidx.compose.foundation.gestures.awaitFirstDown
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.ExperimentalLayoutApi
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.WindowInsets
@@ -20,7 +21,7 @@ import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.ime
-import androidx.compose.foundation.layout.imePadding
+import androidx.compose.foundation.layout.isImeVisible
 import androidx.compose.foundation.layout.navigationBars
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
@@ -211,6 +212,9 @@ private fun Modifier.holdTopEdge(key: Any, held: TopEdgeHold, hold: (Int) -> Uni
 // TranscriptItems.kt: it is pure event folding with no screen in it, and the two halves changed
 // for unrelated reasons while they shared this file.
 
+// isImeVisible: see the comment beside `imeVisible` below for why the keyboard's own
+// self-correction needs it.
+@OptIn(ExperimentalLayoutApi::class)
 @Composable
 fun SessionScreen(settings: ServerSettings, summary: SessionSummary, onBack: () -> Unit) {
     DebugStats.count("session screen recomposed")
@@ -1060,6 +1064,23 @@ fun SessionScreen(settings: ServerSettings, summary: SessionSummary, onBack: ()
     var composerHeight by remember { mutableIntStateOf(0) }
     val imeInsets = WindowInsets.ime
     val navInsets = WindowInsets.navigationBars
+    // Ground truth for whether the keyboard is up, independent of `imeInsets` -- which is what
+    // rescues this from a real fault rather than merely reading the same thing twice. `imeInsets`
+    // is driven by the animation as it interpolates and is dispatched every frame; `isImeVisible`
+    // is dispatched once, from the platform's own start/end of the transition, over a different
+    // path (`onApplyWindowInsets` rather than the animation callback).
+    //
+    // Reported from a phone: closing the keyboard on purpose, while a reply was streaming, left
+    // the composer floating above the bottom of the screen for the rest of the session, with a
+    // bar of background colour showing under it and nothing that closed it. The likely cause is
+    // the animation callback that carries `imeInsets` back to zero being interrupted mid-flight --
+    // a streaming reply invalidates the view every frame, which is exactly the condition known to
+    // starve a running `WindowInsetsAnimationCallback` of its `onEnd` -- and once that happens the
+    // stale, partway value it leaves behind has nothing left to correct it: the keyboard is not
+    // going to move again on its own. `isImeVisible` does not share that failure mode (it is not
+    // 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
     Box(Modifier.fillMaxSize()) {
         Column(Modifier.fillMaxSize()) {
             Row(
@@ -1199,8 +1220,19 @@ fun SessionScreen(settings: ServerSettings, summary: SessionSummary, onBack: ()
                     // message -- and then the keyboard's, per frame of its animation. This modifier
                     // is the whole of what the keyboard re-measures: the box's own size never
                     // changes, so nothing above it is touched.
-                    .padding(bottom = with(LocalDensity.current) { composerHeight.toDp() })
-                    .imePadding()
+                    .padding(
+                        bottom =
+                            with(LocalDensity.current) { composerHeight.toDp() } +
+                                // Not the `imePadding()` modifier: it trusts the same animated
+                                // value `imeVisible` exists to correct, so it is exactly as prone
+                                // to sticking open. Coerced to zero the moment the platform says
+                                // the keyboard is gone, whatever the animation still claims.
+                                if (imeVisible) {
+                                    with(LocalDensity.current) { imeInsets.getBottom(this).toDp() }
+                                } else {
+                                    0.dp
+                                }
+                    )
             ) {
                 Box(Modifier.fillMaxSize()) {
                     TranscriptList(
@@ -1542,10 +1574,17 @@ fun SessionScreen(settings: ServerSettings, summary: SessionSummary, onBack: ()
                 .fillMaxWidth()
                 .onSizeChanged { composerHeight = it.height }
                 .graphicsLayer {
+                    // See `imeVisible` above: a callback interrupted mid-close leaves this stuck
+                    // reading a stale height, and without the guard the composer floats above the
+                    // bottom of the screen for good.
                     translationY =
-                        -(imeInsets.getBottom(this) - navInsets.getBottom(this))
-                            .coerceAtLeast(0)
-                            .toFloat()
+                        if (imeVisible) {
+                            -(imeInsets.getBottom(this) - navInsets.getBottom(this))
+                                .coerceAtLeast(0)
+                                .toFloat()
+                        } else {
+                            0f
+                        }
                 }
                 .background(MaterialTheme.colorScheme.background)
         ) {