fix(web): CropBounds metadata + Fill-style scaling in compositor — transparent margins preserved, widget fills element rect

Services/Compositor/SceneCompositor.cs

@@ -420,9 +420,42 @@ public sealed class SceneCompositor
         VideoFrame src, float opacity, bool isRound, bool isMirror)
     {
         if (ew <= 0 || eh <= 0) return;
-        var (scale, ox, oy) = StretchMath.UniformToFill(ew, eh, src.Width, src.Height);
-        var drawnW = src.Width * scale;
-        var drawnH = src.Height * scale;
+
+        float scale, ox, oy;
+        int srcCropX = 0, srcCropY = 0;
+        int w = src.Width, hgt = src.Height;
+        int sxMax, syMax;
+        float drawnW, drawnH;
+
+        if (src.CropBounds is (var cbX, var cbY, var cbW, var cbH))
+        {
+            // WebSource: Fill-style (stretch to cover, no aspect preservation) using
+            // the actual crop bounds. The transparent margins of the full canvas are
+            // preserved so they reveal layers beneath; only the crop content fills
+            // the element rect — matching the preview's Stretch="Fill" on the crop.
+            scale = Math.Max(ew / cbW, eh / cbH);
+            drawnW = cbW * scale;
+            drawnH = cbH * scale;
+            ox = (ew - drawnW) / 2f;
+            oy = (eh - drawnH) / 2f;
+            srcCropX = cbX;
+            srcCropY = cbY;
+            w = cbW;
+            hgt = cbH;
+        }
+        else
+        {
+            var (uScale, uOx, uOy) = StretchMath.UniformToFill(ew, eh, src.Width, src.Height);
+            scale = uScale;
+            ox = uOx;
+            oy = uOy;
+            drawnW = src.Width * scale;
+            drawnH = src.Height * scale;
+        }
+
+        sxMax = w - 1;
+        syMax = hgt - 1;
+        var invScale = 1f / scale;
         var radius = Math.Min(ew, eh) / 2f;
         var cx = ew / 2f;
         var cy = eh / 2f;
@@ -433,16 +466,6 @@ public sealed class SceneCompositor
         var y1 = Math.Min(dstH - 1, (int)Math.Ceiling(ey + eh));
         if (x0 > x1 || y0 > y1) return;
 
-        // Integer fixed-point sampling + blend (libyuv's shape): the float
-        // SampleBgra + BlendPixel pair cost ~8 Math.Round + ~8 divisions per
-        // pixel and dominated this path in takes 3–4. Row invariants hoisted,
-        // the y-range check moved out of the x loop, and the dst alpha is the
-        // same opaque-master invariant as the row helpers (every write = 255).
-        var w = src.Width;
-        var hgt = src.Height;
-        var sxMax = w - 1;
-        var syMax = hgt - 1;
-        var invScale = 1f / scale;
         var op = (int)(Math.Clamp(opacity, 0f, 1f) * 256);
         if (op <= 0) return;
         var s = src.BgraPixels;
@@ -451,7 +474,7 @@ public sealed class SceneCompositor
         {
             var py = y - ey;
             if (py < oy || py > oy + drawnH) continue;
-            var syRaw = Math.Clamp((py - oy) * invScale, 0f, syMax);
+            var syRaw = Math.Clamp((py - oy) * invScale, 0f, syMax) + srcCropY;
             var syi = (int)syRaw;
             var sy1i = Math.Min(syi + 1, syMax);
             var fy8 = (int)((syRaw - syi) * 256);
@@ -459,7 +482,7 @@ public sealed class SceneCompositor
             var di = (y * dstW + x0) * 4;
             for (var x = x0; x <= x1; x++, di += 4)
             {
-                var px = x - ex; // element space
+                var px = x - ex;
                 if (px < ox || px > ox + drawnW) continue;
                 if (isRound)
                 {
@@ -468,7 +491,7 @@ public sealed class SceneCompositor
                     if (dx * dx + dy * dy > radius * radius) continue;
                 }
 
-                var sxRaw = (px - ox) * invScale;
+                var sxRaw = Math.Clamp((px - ox) * invScale, 0f, sxMax) + srcCropX;
                 if (isMirror) sxRaw = sxMax - sxRaw;
                 sxRaw = Math.Clamp(sxRaw, 0f, sxMax);
                 var sxi = (int)sxRaw;

Services/VideoFrame.cs

@@ -27,6 +27,13 @@ public sealed class VideoFrame
     /// be transparent (static art, web overlays, chat, the social-bar strip).</summary>
     public bool IsOpaque { get; init; }
 
+    /// <summary>WebSource crop metadata: the alpha bounding box of the widget content
+    /// within the full canvas (X/Y/W/H in pixels). When set, the compositor uses these
+    /// bounds for Fill-style scaling (stretch to cover, no aspect preservation) instead
+    /// of UniformToFill. The transparent margins of the full canvas are preserved so
+    /// they reveal layers beneath — the crop defines the actual content region.</summary>
+    public (int X, int Y, int W, int H)? CropBounds { get; init; }
+
     public VideoFrame(int width, int height, byte[] bgraPixels)
     {
         Width = width;

Services/WebView2Manager.cs

@@ -270,7 +270,11 @@ 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(pixW, pixH, pixels) { Epoch = ++session.Epoch };
+            session.LatestFrame = new VideoFrame(pixW, pixH, pixels)
+            {
+                Epoch = ++session.Epoch,
+                CropBounds = (cropX, cropY, cropW, cropH),
+            };
 
             // The preview bitmap is now MUTABLE and reused (CameraManager precedent):
             // WPF re-renders it after WritePixels; handlers must not Freeze it.