Web source: container == content — element grid + selection box sized to the cropped frame's canvas extent

- FindContentBounds already trimmed the bitmap; the grid/box still rendered at the uncropped user
  size, so Fill stretched the tight widget into a bigger frame = dead-between-content-and-border.
- ContentCanvasSize maps the crop rect to canvas units (1920/pixW per axis, DPI-correct) and the
  preview event now carries (bitmap, canvasW, canvasH).
- OnWebView2PreviewBitmapChanged sets the element Width/Height to that size; the grid and the
  SelectionOverlay (both bound to the element's Width/Height) now equal the widget exactly.
- IsPreviewDragging (set in MainWindow drag/resize) suppresses auto-fit mid-gesture.
- Tests 10/10 (alpha bounds, canvas-size mapping incl. DPI, control size); 0 warnings.

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>? PreviewBitmapChanged;
+    public event Action<string, WriteableBitmap, double, double>? PreviewBitmapChanged;
 
     private sealed class WebSourceSession
     {
@@ -182,6 +182,14 @@ 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;
@@ -232,11 +240,12 @@ 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);
+                PreviewBitmapChanged?.Invoke(sourceId, wb, canvasW, canvasH);
             });
         }
         catch (ObjectDisposedException) { }