perf(android): move thumbnail encoding off UI path

apps/weblibre/lib/features/geckoview/features/browser/presentation/widgets/browser_modules/browser_view.dart

@@ -111,8 +111,16 @@ class _BrowserViewState extends ConsumerState<BrowserView>
   static const _pointerThrottleInterval = Duration(milliseconds: 32);
   DateTime _lastPointerEvent = DateTime(0);
   Offset _accumulatedDelta = Offset.zero;
+  bool _screenshotCaptureInFlight = false;
 
   Future<void> _timerTick(Timer timer) async {
+    // Timer.periodic does not await async callbacks. A slow Gecko capture used
+    // to overlap the next tick, multiplying GPU readbacks, bitmap encoders and
+    // thumbnail events exactly while the device was already under load.
+    if (_screenshotCaptureInFlight) {
+      return;
+    }
+
     // Skip the (expensive) Gecko render-to-bitmap while a full-cover route
     // (settings, tab tray, search, …) occludes the browser. The screenshot
     // would force an off-screen render the user can't see and competes for the
@@ -133,15 +141,17 @@ class _BrowserViewState extends ConsumerState<BrowserView>
       return;
     }
 
+    _screenshotCaptureInFlight = true;
+    try {
       await ref
           .read(selectedTabSessionProvider)
-        .requestScreenshot(requireImageResult: false)
+          .requestScreenshot(requireImageResult: false);
-        .onError((error, stackTrace) {
+    } catch (error, stackTrace) {
       logger.e(error, stackTrace: stackTrace);
       timer.cancel();
-
+    } finally {
-          return null;
+      _screenshotCaptureInFlight = false;
-        });
+    }
   }
 
   @override

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/ext/Bitmap.kt

@@ -9,32 +9,42 @@ package eu.weblibre.flutter_mozilla_components.ext
 import android.graphics.Bitmap
 import android.os.Build
 import java.io.ByteArrayOutputStream
+import kotlin.math.roundToInt
 
 fun Bitmap.resize(maxWidth: Int, maxHeight: Int): Bitmap {
-    var width = this.width
+    require(maxWidth > 0 && maxHeight > 0) {
-    var height = this.height
+        "Bitmap bounds must be positive"
-
-    val aspectRatio: Float = width.toFloat() / height.toFloat()
-
-    if (width > height) {
-        width = maxWidth
-        height = (width / aspectRatio).toInt()
-    } else {
-        height = maxHeight
-        width = (height * aspectRatio).toInt()
     }
 
-    return Bitmap.createScaledBitmap(this, width, height, true)
+    if (width <= maxWidth && height <= maxHeight) {
+        return this
     }
 
-fun Bitmap.toWebPBytes(): ByteArray {
+    val scale = minOf(
+        maxWidth.toFloat() / width.toFloat(),
+        maxHeight.toFloat() / height.toFloat(),
+    )
+    val targetWidth = (width * scale).roundToInt().coerceAtLeast(1)
+    val targetHeight = (height * scale).roundToInt().coerceAtLeast(1)
+
+    return Bitmap.createScaledBitmap(this, targetWidth, targetHeight, true)
+}
+
+fun Bitmap.toWebPBytes(
+    lossless: Boolean = true,
+    quality: Int = 100,
+): ByteArray {
     val stream = ByteArrayOutputStream()
     val compressFormat = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+        if (lossless) {
             Bitmap.CompressFormat.WEBP_LOSSLESS
+        } else {
+            Bitmap.CompressFormat.WEBP_LOSSY
+        }
     } else {
         @Suppress("DEPRECATION")
         Bitmap.CompressFormat.WEBP
     }
-    compress(compressFormat, 100, stream)
+    compress(compressFormat, quality.coerceIn(0, 100), stream)
     return stream.toByteArray()
 }

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/middleware/FlutterEventMiddleware.kt

@@ -19,6 +19,15 @@ import eu.weblibre.flutter_mozilla_components.pigeons.ImageSrcHitResult
 import eu.weblibre.flutter_mozilla_components.pigeons.PhoneHitResult
 import eu.weblibre.flutter_mozilla_components.pigeons.UnknownHitResult
 import eu.weblibre.flutter_mozilla_components.pigeons.VideoHitResult
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.isActive
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import mozilla.components.browser.state.action.BrowserAction
 import mozilla.components.browser.state.action.ContentAction
 import mozilla.components.browser.state.action.LastAccessAction
@@ -45,6 +54,53 @@ class FlutterEventMiddleware(private val flutterEvents: GeckoStateEvents) : Midd
         requireNotNull(GlobalComponents.components) { "Components not initialized" }
     }
 
+    /**
+     * Thumbnail scaling and WebP encoding are CPU-heavy and this middleware is
+     * normally invoked on the browser/UI dispatch path. Keep that work off the
+     * frame-critical thread and serialize it so periodic captures cannot build
+     * up a queue of competing bitmap encoders.
+     */
+    private val thumbnailEncodingScope = CoroutineScope(SupervisorJob() + Dispatchers.Default)
+    private val thumbnailEncodingMutex = Mutex()
+    private var thumbnailEncodingJob: Job? = null
+
+    private fun forwardThumbnail(action: ContentAction.UpdateThumbnailAction) {
+        // Only the newest selected-tab preview is useful. Cancellation does not
+        // interrupt Bitmap.compress itself, so the mutex also prevents a newer
+        // request from starting a second encoder before the old one unwinds.
+        thumbnailEncodingJob?.cancel()
+        thumbnailEncodingJob = thumbnailEncodingScope.launch {
+            thumbnailEncodingMutex.withLock {
+                if (!isActive) return@withLock
+
+                val resized = action.thumbnail.resize(maxWidth = 720, maxHeight = 720)
+                try {
+                    // Thumbnails are displayed at a few hundred logical pixels;
+                    // lossless 1280x800 WebP added CPU and channel traffic with
+                    // no visible benefit.
+                    val bytes = resized.toWebPBytes(lossless = false, quality = 82)
+                    if (!isActive) return@withLock
+
+                    runOnUiThread {
+                        flutterEvents.onThumbnailChange(
+                            EventSequence.next(),
+                            action.sessionId,
+                            bytes,
+                        ) { _ -> }
+                    }
+                } catch (error: CancellationException) {
+                    throw error
+                } catch (error: Exception) {
+                    Log.e("FlutterEventMiddleware", "Failed to encode thumbnail", error)
+                } finally {
+                    if (resized !== action.thumbnail && !resized.isRecycled) {
+                        resized.recycle()
+                    }
+                }
+            }
+        }
+    }
+    
     @Suppress("ComplexMethod")
     override fun invoke(
         store: Store<BrowserState, BrowserAction>,
@@ -53,12 +109,7 @@ class FlutterEventMiddleware(private val flutterEvents: GeckoStateEvents) : Midd
     ) {
         when (action) {
             is ContentAction.UpdateThumbnailAction -> {
-                val resized = action.thumbnail.resize(maxWidth = 1280, maxHeight = 800);
+                forwardThumbnail(action)
-                val bytes = resized.toWebPBytes()
-
-                runOnUiThread {
-                    flutterEvents.onThumbnailChange(EventSequence.next(), action.sessionId, bytes) { _ -> }
-                }
             }
             //UpdateReaderConnectRequiredAction seems to be the only event that is called predictable
             //after a hot reload