fix: first native launch since refactor crashed — 3 stacked faults, all closed; RoundClip 'known failure' root-caused and green

User launch (2026-09-01 19:07) NRE'd in MainViewModel ctor:
1. SceneGraph (TASK 31) was 'null!'-declared, assigned mid-ctor, but Scenes is touched
   ~120 lines earlier — field-initialized now.
2. LeftPanel extraction (c9fd1bd) moved StaticResource users (EyeButton/EyeIconStyle)
   into a UserControl while the styles stayed window-scope — invisible at parse time;
   moved to Themes/Controls.xaml (app scope, the existing rule). Full audit: these were
   the only two offenders (grep of Controls/*.xaml StaticResource keys vs app dictionary).
3. RoundClipInteractionTests — the second 'known failure' the map never explained: it
   was two stale-test layers (window.FindName across the new UserControl namescope +
   VisualTreeHelper.HitTest, which returned the IsHitTestVisible=False WebViewHostPanel
   overlay for EVERY point; UIElement.InputHitTest — the real input pipeline — shows the
   corner IS grabbable in both Traditional and Round). Test fixed, no product bug.
Verified: clean rebuild 0 warnings; app boots (log shows full MainWindow loaded; user
clicked + closed, zero new exceptions; real DB Webcam row = the genuine C920, untouched);
RoundClip + 6 RealApp classes pass natively per-class.
Docs: ai.md known-failure note → 246/247 (audio only); TASK 31 verification paragraph
corrected ('cannot run headless' overstated — per-class Windows-host vstest runs them);
MyMistakes: InputHitTest-vs-VTH recipe + namescope/app-style + shared-log facts.
Spin-guard citations: WPF Visual Tree Overview (InputHitTest vs VisualTreeHelper hit
semantics) + XAML namescope docs, learn.microsoft.com.

ytLive.Tests/RoundClipInteractionTests.cs

@@ -34,6 +34,7 @@ public sealed class RoundClipInteractionTests
         _app.Run(Run);
     }
 
+
     private void Run()
     {
         // Never let the real MainWindow read/write the user's actual layout DB —
@@ -62,8 +63,12 @@ public sealed class RoundClipInteractionTests
             vm.SelectedElement = webcam;
             window.UpdateLayout();
 
-            var previewGrid = (Grid)window.FindName("PreviewGrid")!;
-            var canvasGrid = (Grid)window.FindName("CanvasGrid")!;
+            // PreviewGrid/CanvasGrid moved into Controls/PreviewPane.xaml during the
+            // component refactor — a UserControl owns its own namescope, so window.FindName
+            // returns null for them. Resolve through the pane (found by its window-level name).
+            var pane = (System.Windows.Controls.UserControl)window.FindName("PreviewPane")!;
+            var previewGrid = (Grid)pane.FindName("PreviewGrid")!;
+            var canvasGrid = (Grid)pane.FindName("CanvasGrid")!;
             var toWindow = canvasGrid.TransformToVisual(window);
 
             var corner = new Point(webcam.X + webcam.Width, webcam.Y + webcam.Height);
@@ -78,11 +83,16 @@ public sealed class RoundClipInteractionTests
                 // A click on the corner must land inside the preview (descendant
                 // of PreviewGrid); otherwise Window_PreviewMouseLeftButtonDown
                 // deselects the source and the handle can never be grabbed.
-                var hit = VisualTreeHelper.HitTest(window, cornerInWindow);
-                var grabbable = hit != null && IsDescendantOf(hit.VisualHit, previewGrid);
+                // UIElement.InputHitTest IS the input pipeline — what Mouse uses.
+                // Do NOT use VisualTreeHelper.HitTest here: it returned the window's
+                // non-hit-testable WebViewHostPanel overlay for EVERY point (it does
+                // not model IsHitTestVisible input semantics) — the bug this test
+                // "found" for months was this wrong API (root-caused 2026-09-01).
+                var hit = window.InputHitTest(cornerInWindow) as DependencyObject;
+                var grabbable = hit != null && IsDescendantOf(hit, previewGrid);
 
                 Assert.True(grabbable,
-                    $"{shape}: corner click fell through to '{hit?.VisualHit.GetType().Name ?? "null"}' " +
+                    $"{shape}: corner click fell through to '{hit?.GetType().Name ?? "null"}' " +
                     "so the source gets deselected before the resize handler runs");
             }