fix(web): feed full canvas to compositor — transparent margins reveal layers beneath (take-21)

Services/WebView2Manager.cs

@@ -30,13 +30,13 @@ public sealed class WebView2Manager : IDisposable
         public bool Disposed;
         public bool Initialized;
 
-        // Buffer reuse (2026-09-04, same lesson as the capture/camera rings): every
+        // Buffer reuse (2026-09-04): every capture tick used to mint a fresh
-        // capture tick used to mint a fresh 8.3MB canvas array + a fresh crop array
+        // 8.3MB canvas array + a fresh crop array (~83MB/s of LOH at 10Hz →
-        // (~83MB/s of LOH at 10Hz → gen2 pauses surfaced as the pump's 35-40ms
+        // gen2 pauses surfaced as the pump's 35-40ms "worst render" spikes).
-        // "worst render" spikes). Canvas scratch is reused while the size holds;
+        // CanvasScratch is reused while the size holds. The frame handed to the
-        // the frame handed to the compositor rotates an 8-deep ring with a monotonic
+        // compositor is always the FULL canvas (transparent margins reveal layers
-        // Epoch (identity-keyed consumers must see each generation). The preview
+        // beneath); the crop is only for the preview WriteableBitmap. Epoch is
-        // WriteableBitmap is recreated only when the cropped size changes.
+        // monotonic per canvas generation (identity-keyed consumers see each frame).
         public byte[]? CanvasScratch;
         public readonly byte[]?[] OutRing = new byte[8][];
         public int OutRingNext;
@@ -192,9 +192,13 @@ public sealed class WebView2Manager : IDisposable
     // bounding box of non-transparent pixels (the background is injected
     // transparent, so the alpha channel IS the widget). This is immune to
     // DOM timing (fonts/images/animations), needs no JS round-trip, and
-    // CANNOT break rendering: if the widget fills the canvas the box equals
+    // CANNOT break the render: if the widget fills the canvas the box equals
-    // the full frame (identical to the confirmed-good uncropped display); if
+    // the full frame; if it floats in transparent margin the box is the
-    // it floats in transparent margin, the crop maps it flush under the box.
+    // measured content extent — used ONLY for the preview bitmap / selection.
+    // The frame handed to the compositor is always the FULL canvas, so the
+    // page's transparent margins keep revealing the layers beneath (the
+    // "black square over webcam" was the crop fed to the compositor:
+    // UniformToFill zoomed the opaque content box to cover the whole element).
     internal static (int X, int Y, int W, int H) FindContentBounds(byte[] pixels, int pixW, int pixH, int stride)
     {
         int minX = pixW, minY = pixH, maxX = -1, maxY = -1;
@@ -266,7 +270,7 @@ public sealed class WebView2Manager : IDisposable
                 session.PreviewBitmap = new WriteableBitmap(cropW, cropH, 96, 96, PixelFormats.Bgra32, null);
             session.PreviewBitmap.WritePixels(new Int32Rect(0, 0, cropW, cropH), outPixels, cropW * 4, 0);
 
-            session.LatestFrame = new VideoFrame(cropW, cropH, outPixels) { Epoch = ++session.Epoch };
+            session.LatestFrame = new VideoFrame(pixW, pixH, pixels) { Epoch = ++session.Epoch };
 
             // The preview bitmap is now MUTABLE and reused (CameraManager precedent):
             // WPF re-renders it after WritePixels; handlers must not Freeze it.