Make image viewer truly full screen

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

@@ -10,14 +10,20 @@ import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.navigationBarsPadding
+import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
+import androidx.compose.material3.Button
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.SideEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
@@ -32,12 +38,20 @@ import androidx.compose.ui.graphics.asImageBitmap
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.ContentScale
+import androidx.compose.ui.layout.onSizeChanged
 import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalView
 import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.IntSize
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.isSpecified
 import androidx.compose.ui.window.Dialog
 import androidx.compose.ui.window.DialogProperties
+import androidx.compose.ui.window.DialogWindowProvider
+import androidx.core.view.ViewCompat
+import androidx.core.view.WindowCompat
+import androidx.core.view.WindowInsetsCompat
+import androidx.core.view.WindowInsetsControllerCompat
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 
@@ -142,10 +156,15 @@ fun SessionImageViewer(
     onClose: () -> Unit,
 ) {
     val (bitmap, failed) = rememberSessionBitmap(settings, sessionId, ref)
+    val view = LocalView.current
+    var hiddenBars by remember(ref) { mutableStateOf(ViewerBars()) }
+    var barInsets by remember(ref) { mutableStateOf(ViewerBarInsets()) }
     Dialog(
         onDismissRequest = onClose,
-        properties = DialogProperties(usePlatformDefaultWidth = false),
+        properties =
+            DialogProperties(usePlatformDefaultWidth = false, decorFitsSystemWindows = false),
     ) {
+        ViewerSystemBars(hiddenBars)
         Box(
             Modifier.fillMaxSize()
                 .background(Color.Black)
@@ -173,7 +192,46 @@ fun SessionImageViewer(
                         // beside it are.
                         CircularProgressIndicator(color = Color.White)
                     }
-                else -> ZoomableImage(image)
+                else -> {
+                    var viewport by remember { mutableStateOf(IntSize.Zero) }
+                    var nativeSizeRequest by remember { mutableIntStateOf(0) }
+                    ZoomableImage(
+                        image,
+                        nativeSizeRequest = nativeSizeRequest,
+                        onViewportChanged = {
+                            viewport = it
+                            ViewCompat.getRootWindowInsets(view)?.let { insets ->
+                                barInsets =
+                                    ViewerBarInsets(
+                                        status =
+                                            insets
+                                                .getInsetsIgnoringVisibility(
+                                                    WindowInsetsCompat.Type.statusBars()
+                                                )
+                                                .top,
+                                        navigation =
+                                            insets
+                                                .getInsetsIgnoringVisibility(
+                                                    WindowInsetsCompat.Type.navigationBars()
+                                                )
+                                                .bottom,
+                                    )
+                            }
+                        },
+                        onBarsChanged = { hiddenBars = it },
+                        barInsets = barInsets,
+                        viewport = viewport,
+                    )
+                    Button(
+                        onClick = { nativeSizeRequest++ },
+                        modifier =
+                            Modifier.align(Alignment.BottomEnd)
+                                .navigationBarsPadding()
+                                .padding(16.dp),
+                    ) {
+                        Text("100%")
+                    }
+                }
             }
         }
     }
@@ -233,14 +291,40 @@ private fun enlargingFilter(sourceHeight: Int, drawnHeight: Int): FilterQuality
  * The image on its own, as large as it fits, with pinch to zoom.
  *
  * 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.
+ * gesture returns to the transcript instead of leaving the app. It opens fitted, with the whole
+ * image visible; the 100% control changes to one bitmap pixel per screen pixel and recenters it.
  */
 @Composable
-private fun ZoomableImage(image: ImageBitmap) {
+private fun ZoomableImage(
+    image: ImageBitmap,
+    nativeSizeRequest: Int,
+    onViewportChanged: (IntSize) -> Unit,
+    onBarsChanged: (ViewerBars) -> Unit,
+    barInsets: ViewerBarInsets,
+    viewport: IntSize,
+) {
     var scale by remember { mutableFloatStateOf(1f) }
     var offsetX by remember { mutableFloatStateOf(0f) }
     var offsetY by remember { mutableFloatStateOf(0f) }
+    val nativeScale = nativeScale(image.width, image.height, viewport.width, viewport.height)
+    LaunchedEffect(nativeSizeRequest, nativeScale) {
+        if (nativeSizeRequest > 0) {
+            scale = nativeScale
+            offsetX = 0f
+            offsetY = 0f
+        }
+    }
+    val bars =
+        viewerBars(
+            image.width,
+            image.height,
+            viewport.width,
+            viewport.height,
+            scale,
+            Offset(offsetX, offsetY),
+            barInsets,
+        )
+    SideEffect { onBarsChanged(bars) }
     Image(
         bitmap = image,
         contentDescription = "Attached image",
@@ -249,12 +333,13 @@ private fun ZoomableImage(image: ImageBitmap) {
         filterQuality = FilterQuality.None,
         modifier =
             Modifier.fillMaxSize()
-                .pointerInput(Unit) {
+                .onSizeChanged(onViewportChanged)
+                .pointerInput(nativeScale) {
                     detectTransformGestures { centroid, pan, zoom, _ ->
-                        // Floor of 1 so the image cannot be pinched smaller than fitted, which is
-                        // already the whole of it; a ceiling so it cannot be lost off-screen.
                         val oldScale = scale
-                        val newScale = (oldScale * zoom).coerceIn(1f, 8f)
+                        val minimumScale = minOf(1f, nativeScale)
+                        val maximumScale = maxOf(8f, nativeScale)
+                        val newScale = (oldScale * zoom).coerceIn(minimumScale, maximumScale)
                         if (newScale > 1f) {
                             val offset =
                                 zoomOffset(
@@ -283,6 +368,84 @@ private fun ZoomableImage(image: ImageBitmap) {
     )
 }
 
+/** Lets the picture use the whole display, hiding only the system bars it actually reaches. */
+@Composable
+private fun ViewerSystemBars(hidden: ViewerBars) {
+    val view = LocalView.current
+    val window = (view.parent as? DialogWindowProvider)?.window
+    val controller = window?.let { WindowCompat.getInsetsController(it, view) }
+    SideEffect {
+        controller?.systemBarsBehavior =
+            WindowInsetsControllerCompat.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE
+        if (hidden.status) {
+            controller?.hide(WindowInsetsCompat.Type.statusBars())
+        } else {
+            controller?.show(WindowInsetsCompat.Type.statusBars())
+        }
+        if (hidden.navigation) {
+            controller?.hide(WindowInsetsCompat.Type.navigationBars())
+        } else {
+            controller?.show(WindowInsetsCompat.Type.navigationBars())
+        }
+    }
+    DisposableEffect(view) {
+        onDispose {
+            controller?.show(
+                WindowInsetsCompat.Type.statusBars() or WindowInsetsCompat.Type.navigationBars()
+            )
+        }
+    }
+}
+
+internal data class ViewerBars(val status: Boolean = false, val navigation: Boolean = false)
+
+internal data class ViewerBarInsets(val status: Int = 0, val navigation: Int = 0)
+
+/** Which full-screen system-bar regions the fitted, zoomed and panned image intersects. */
+internal fun viewerBars(
+    imageWidth: Int,
+    imageHeight: Int,
+    viewportWidth: Int,
+    viewportHeight: Int,
+    scale: Float,
+    offset: Offset,
+    insets: ViewerBarInsets,
+): ViewerBars {
+    if (imageWidth <= 0 || imageHeight <= 0 || viewportWidth <= 0 || viewportHeight <= 0) {
+        return ViewerBars()
+    }
+    val fittedScale =
+        minOf(viewportWidth.toFloat() / imageWidth, viewportHeight.toFloat() / imageHeight)
+    val width = imageWidth * fittedScale * scale
+    val height = imageHeight * fittedScale * scale
+    val left = viewportWidth / 2f + offset.x - width / 2f
+    val right = left + width
+    val top = viewportHeight / 2f + offset.y - height / 2f
+    val bottom = top + height
+    val crossesScreen = right > 0f && left < viewportWidth
+    return ViewerBars(
+        status = crossesScreen && insets.status > 0 && bottom > 0f && top < insets.status,
+        navigation =
+            crossesScreen &&
+                insets.navigation > 0 &&
+                bottom > viewportHeight - insets.navigation &&
+                top < viewportHeight,
+    )
+}
+
+/** Scale relative to [ContentScale.Fit] at which bitmap and screen pixels are one-to-one. */
+internal fun nativeScale(
+    imageWidth: Int,
+    imageHeight: Int,
+    viewportWidth: Int,
+    viewportHeight: Int,
+): Float {
+    if (imageWidth <= 0 || imageHeight <= 0 || viewportWidth <= 0 || viewportHeight <= 0) return 1f
+    val fittedScale =
+        minOf(viewportWidth.toFloat() / imageWidth, viewportHeight.toFloat() / imageHeight)
+    return 1f / fittedScale
+}
+
 /** Keeps the image point beneath [centroid] beneath the fingers as its scale changes. */
 internal fun zoomOffset(
     offset: Offset,