Revert ab29ec8: container==content auto-fit made it worse — back to alpha-crop with the container at element size

The container tracks the element's own Width/Height again; the crop stays in the bitmap.

Services/WebView2Manager.cs

@@ -20,7 +20,7 @@ public sealed class WebView2Manager : IDisposable
     private readonly Dispatcher _dispatcher;
     private readonly Dictionary<string, WebSourceSession> _sessions = new();
 
-    public event Action<string, WriteableBitmap, double, double>? PreviewBitmapChanged;
+    public event Action<string, WriteableBitmap>? PreviewBitmapChanged;
 
     private sealed class WebSourceSession
     {
@@ -182,14 +182,6 @@ public sealed class WebView2Manager : IDisposable
         return (minX, minY, maxX - minX + 1, maxY - minY + 1);
     }
 
-    // The cropped pixel extent, expressed in MASTER CANVAS units (the WebView2 is
-    // 1920×1080 DIPs and captures at its physical size, so 1920/pixW maps capture
-    // pixels back to canvas units). The element grid + selection box are sized to
-    // this, so the container matches the content exactly instead of Fill-stretching
-    // a cropped bitmap into a larger frame.
-    internal static (double W, double H) ContentCanvasSize(int cropW, int cropH, int pixW, int pixH)
-        => (cropW * (1920.0 / pixW), cropH * (1080.0 / pixH));
-
     private async Task CaptureFrame(string sourceId)
     {
         if (!_sessions.TryGetValue(sourceId, out var session) || session.Disposed) return;
@@ -240,12 +232,11 @@ public sealed class WebView2Manager : IDisposable
             wb.WritePixels(new Int32Rect(0, 0, cropW, cropH), outPixels, cropW * 4, 0);
             wb.Freeze();
 
-            var (canvasW, canvasH) = ContentCanvasSize(cropW, cropH, pixW, pixH);
             session.LatestFrame = new VideoFrame(cropW, cropH, outPixels);
 
             _ = _dispatcher.BeginInvoke(() =>
             {
-                PreviewBitmapChanged?.Invoke(sourceId, wb, canvasW, canvasH);
+                PreviewBitmapChanged?.Invoke(sourceId, wb);
             });
         }
         catch (ObjectDisposedException) { }