Fix: Elements scrollbar click no longer deselects the selected element

The scrollbar lives inside the ScrollViewer, not the StackPanel
(ElementsPanel). Window_PreviewMouseLeftButtonDown was checking
IsDescendantOf against ElementsPanel, so scrollbar clicks fell through
and nulled SelectedElement. Now checks ElementsScrollViewer instead.

1a of 7-issue polish batch. 207 tests, 0 warnings.

MainWindow.xaml

@@ -366,7 +366,7 @@
                                 Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}">
                         <TextBlock Text="ELEMENTS" Style="{StaticResource SectionHeader}" VerticalAlignment="Center"/>
                     </StackPanel>
-                    <ScrollViewer Grid.Row="7" VerticalScrollBarVisibility="Auto" Background="Transparent"
+                    <ScrollViewer x:Name="ElementsScrollViewer" Grid.Row="7" VerticalScrollBarVisibility="Auto" Background="Transparent"
                                   Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}">
                         <StackPanel x:Name="ElementsPanel" Margin="0,4,0,0">
                             <!-- Opacity + mirror/shape (draggable elements: image, webcam) -->

MainWindow.xaml.cs

@@ -209,7 +209,7 @@ public partial class MainWindow : Window
         if (e.OriginalSource is not DependencyObject original) return;
         if (IsDescendantOf(original, PreviewGrid)) return;
         if (IsDescendantOf(original, SourceList)) return;
-        if (IsDescendantOf(original, ElementsPanel)) return;
+        if (IsDescendantOf(original, ElementsScrollViewer)) return;
         _viewModel.SelectedElement = null;
     }
 

TASKS.md

@@ -825,6 +825,7 @@ No changes to `MusicPlayer`, the 0.20 cap, the ducker, or the meter zones. Build
 8. ✅ Webcam border config — color hex input + color swatch picker, border thickness 0–10 slider
 9. ✅ Countdown source — `SourceType.Countdown`, pick list (Starting/BRB scenes only), timer minutes 1–60 default 15
 10. ✅ Web source — `SourceType.WebSource`, pick list (all scenes), URI text input
+11. ✅ Elements scrollbar deselect fix — clicking the Elements panel scrollbar no longer deselects the selected element (scrollbar lives inside the ScrollViewer, not the StackPanel; guard now checks `ElementsScrollViewer` instead of `ElementsPanel`)
 
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