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.

ViewModels/MainViewModel.cs

@@ -2290,6 +2290,11 @@ public class MainViewModel : ViewModelBase
         _ = RedesignateBackgroundAsync($"{MonitorKeyPrefix}{display.Index}");
     }
 
+    // True while the user is dragging/resizing an element on the preview canvas
+    // (set by MainWindow code-behind). Suppresses the web source's auto-fit so it
+    // doesn't fight the gesture mid-drag.
+    public bool IsPreviewDragging { get; set; }
+
     public void InitWebView2(Panel hostPanel)
     {
         _webView2Manager = new WebView2Manager(hostPanel, System.Windows.Application.Current.Dispatcher);
@@ -2297,11 +2302,26 @@ public class MainViewModel : ViewModelBase
         RegisterLoadedWebSources();
     }
 
-    private void OnWebView2PreviewBitmapChanged(string sourceId, WriteableBitmap bitmap)
+    private void OnWebView2PreviewBitmapChanged(string sourceId, WriteableBitmap bitmap, double canvasW, double canvasH)
     {
         foreach (var scene in Scenes)
             foreach (var el in scene.Elements.OfType<Source>().Where(s => s.Id == sourceId))
+            {
                 el.VideoImageSource = bitmap;
+
+                // The container must equal the content: size the element grid
+                // (and therefore the selection box) to the cropped frame's canvas
+                // extent. Skipped while the user is dragging/resizing the element
+                // so the gesture isn't fought mid-drag.
+                if (IsPreviewDragging || canvasW < 1 || canvasH < 1) continue;
+                var w = Math.Clamp(Math.Round(canvasW), 1, 1920);
+                var h = Math.Clamp(Math.Round(canvasH), 1, 1080);
+                if (Math.Abs(el.Width - w) > 1 || Math.Abs(el.Height - h) > 1)
+                {
+                    el.Width = w;
+                    el.Height = h;
+                }
+            }
     }
 
     public void Shutdown()