Fix #15 (round 2): bind finding-pane visibility to explicit bool (#19)

The Click handler from the previous commit fired correctly but the pane
still didn't collapse, because the visibility binding
`{x:Bind local:MainPage.NullToCollapsed(ViewModel.SelectedFinding)}`
wasn't re-evaluating reliably after the property cleared.

- New observable bool MainPageViewModel.IsFindingDetailVisible, kept in
  sync with SelectedFinding via a partial OnSelectedFindingChanged hook.
- Pane Visibility now binds to BoolToVisibility(IsFindingDetailVisible).
- Close handler also sets IsFindingDetailVisible = false explicitly.
- BtnCloseFinding bumped to 32x32 with bigger glyph for a reliable hit
  target.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclinick

MSIXplainer/MainPage.xaml

@@ -511,20 +511,21 @@
 
                         <!-- ── FINDING DETAIL PANE (shared, right side) ── -->
                         <Border Grid.Column="1" Width="380"
-                                Visibility="{x:Bind local:MainPage.NullToCollapsed(ViewModel.SelectedFinding), Mode=OneWay}"
+                                Visibility="{x:Bind local:MainPage.BoolToVisibility(ViewModel.IsFindingDetailVisible), Mode=OneWay}"
                                 Background="{ThemeResource LayerFillColorDefaultBrush}"
                                 BorderBrush="{ThemeResource CardStrokeColorDefaultBrush}"
                                 BorderThickness="1,0,0,0">
                             <ScrollViewer Padding="20,16,20,24">
                                 <StackPanel Spacing="16">
                                     <!-- Close button -->
                                     <Button AutomationProperties.AutomationId="BtnCloseFinding"
+                                            AutomationProperties.Name="Close finding details"
                                             HorizontalAlignment="Right"
                                             Click="OnCloseFindingClick"
-                                            Padding="4" MinWidth="0" MinHeight="0"
+                                            Padding="8" MinWidth="32" MinHeight="32"
                                             Background="Transparent" BorderThickness="0"
                                             ToolTipService.ToolTip="Close">
-                                        <FontIcon Glyph="&#xE711;" FontSize="12" />
+                                        <FontIcon Glyph="&#xE711;" FontSize="14" />
                                     </Button>
 
                                     <!-- Header -->

MSIXplainer/MainPage.xaml.cs

@@ -200,12 +200,15 @@ private void ViewFinding_Click(object sender, RoutedEventArgs e)
     /// <summary>
     /// Closes the finding detail pane. Clears the OverviewFindingsList selection
     /// explicitly first so the TwoWay binding can't immediately re-push the old
-    /// selection back into ViewModel.SelectedFinding once we null it.
+    /// selection back into ViewModel.SelectedFinding once we null it. Then
+    /// explicitly toggles IsFindingDetailVisible so the pane collapses even if
+    /// the SelectedFinding PropertyChanged notification gets coalesced.
     /// </summary>
     private void OnCloseFindingClick(object sender, RoutedEventArgs e)
     {
         OverviewFindingsList.SelectedItem = null;
         ViewModel.SelectedFinding = null;
+        ViewModel.IsFindingDetailVisible = false;
     }
 
     /// <summary>

MSIXplainer/ViewModels/MainPageViewModel.cs

@@ -25,6 +25,22 @@ public partial class MainPageViewModel : ObservableObject
     [ObservableProperty]
     public partial ManifestFinding? SelectedFinding { get; set; }
 
+    /// <summary>
+    /// Whether the right-side finding-detail pane should be visible. Tracked as a
+    /// distinct observable bool (rather than binding visibility to a
+    /// NullToCollapsed function on SelectedFinding) because the x:Bind function
+    /// re-evaluation was unreliable in combination with the Overview ListView's
+    /// TwoWay SelectedItem binding — the pane would stay open after the close
+    /// button cleared the selection. See issue #15.
+    /// </summary>
+    [ObservableProperty]
+    public partial bool IsFindingDetailVisible { get; set; }
+
+    partial void OnSelectedFindingChanged(ManifestFinding? value)
+    {
+        IsFindingDetailVisible = value is not null;
+    }
+
     [ObservableProperty]
     public partial string AssessmentMessage { get; set; } = string.Empty;