#79 Modern: adopt CornerRadius clip-escape workaround, drop status-bar backplate (#135)

The modern editor's entries ListView hits a WinUI 3 raster clip-escape bug
(upstream microsoft-ui-xaml#11225): once content extent passes ~2^21 DIPs the
renderer stops clipping bottom-edge rows and they paint over the status bar.

The previous mitigation was an opaque backplate behind the status bar
(StatusBackplateThreshold/StatusBackplateVisibility + a Border) that merely
occluded the strays — cosmetic, and it dropped the true-Mica look on large files.

Two responders on the upstream issue proposed a cleaner fix: give the ListView
any non-zero CornerRadius, which forces a composition geometry clip on its
content and re-establishes the bottom clip. CornerRadius="0.1" is visually
indistinguishable from square and actually keeps rows inside the control, so it
works with the transparent Mica band and no backplate is needed.

- Add CornerRadius="0.1" to EntriesList.
- Remove StatusBackplateThreshold, StatusBackplateVisibility, the backplate
  Border, and the two OnPropertyChanged(nameof(StatusBackplateVisibility)) calls.
- Update the surrounding comments.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: scottlerch

HostsFileEditor.WinUI/MainWindow.xaml

@@ -351,9 +351,19 @@
                     <TextBlock Text="{x:Bind SelectAllBannerText, Mode=OneWay}" TextWrapping="Wrap"
                                Foreground="{ThemeResource TextOnAccentFillColorPrimaryBrush}"/>
                 </Border>
+                <!-- CornerRadius="0.1" is the fix for the WinUI raster clip-escape bug (issue #79,
+                     upstream microsoft-ui-xaml#11225): once the list's content extent passes ~2^21
+                     DIPs the renderer stops clipping bottom-edge rows and they paint past the list
+                     into the status bar below. Giving the ListView any non-zero CornerRadius forces
+                     a composition geometry clip on its content, which re-establishes the bottom clip
+                     and keeps the strays inside the control. 0.1 is visually indistinguishable from
+                     square. This actually clips (unlike the old opaque backplate, which only occluded
+                     the strays), so it works with the transparent Mica band. Remove once the upstream
+                     bug ships fixed in a Windows App SDK. -->
                 <ListView x:Name="EntriesList"
                           Grid.Row="1"
                           Margin="0,8,0,0"
+                          CornerRadius="0.1"
                           IsEnabled="{x:Bind IsEntriesInteractive, Mode=OneWay}"
                           PreviewKeyDown="OnEntriesPreviewKeyDown"
                           SelectionMode="Extended"
@@ -470,23 +480,12 @@
              window's Mica backdrop; it sits in its own fixed-height grid row, so no list content is
              ever laid out behind it.
 
-             The backplate underneath exists because of a WinUI composition bug with enormous
-             virtualized lists: past roughly 2^20 device-independent pixels of panel extent
-             (~32K rows here), the renderer stops clipping the list's bottom-edge rows — the last
-             partially-visible row paints in FULL, plus the next cache row — straight through the
-             ListView's viewport clip, an explicit ancestor UIElement.Clip,
-             CanContentRenderOutsideBounds=False, and CacheLength=0 (all verified ineffective;
-             element bounds via UIA stay correct, so it is a raster-level escape, not layout). The
-             stray pixels land under this transparent band and show through it. They only ever paint
-             DOWNWARD past the list (title/command bars stay clean), and opaque content drawn later
-             in the tree occludes them — so when the list is large enough for the bug to trigger,
-             an opaque backplate in SolidBackgroundFillColorBase (the exact base color Mica tints,
-             i.e. "Mica minus the desktop tint") slides in behind the text. Normal-sized files
-             (≤10K rows, empirically clean with margin: breakage sets in between 10K and 50K) keep
-             the true-Mica look. -->
+             The WinUI raster clip-escape bug that used to spill list rows onto this band (issue #79,
+             upstream microsoft-ui-xaml#11225) is now held off at the source by the ListView's
+             CornerRadius="0.1" — see the comment on EntriesList above. That actually clips the rows,
+             so no opaque backplate is needed here and the true-Mica look is preserved at every list
+             size. -->
         <Grid Grid.Row="3" Visibility="{x:Bind MainViewVisibility, Mode=OneWay}">
-            <Border Background="{ThemeResource SolidBackgroundFillColorBaseBrush}"
-                    Visibility="{x:Bind StatusBackplateVisibility, Mode=OneWay}"/>
             <Border Background="Transparent" Padding="14,5">
                 <Grid>
                     <TextBlock Text="{x:Bind StatusText, Mode=OneWay}"

HostsFileEditor.WinUI/MainWindow.xaml.cs

@@ -178,22 +178,6 @@ private void OnPingActivityChanged(object? sender, EventArgs e)
         OnPropertyChanged(nameof(PingProgressVisibility));
     }
 
-    // Above this many visible rows (at 100% scale), show the opaque backplate behind the status
-    // bar. WinUI stops clipping the bottom-edge rows of an enormous virtualized list (see the XAML
-    // comment on the status-bar Grid for the full investigation) and the strays would show through
-    // the transparent Mica band. Empirically clean at 10K rows and broken between 10K and 50K —
-    // measured at 200% scale only, which cannot distinguish whether the underlying limit is
-    // DIP- or physical-pixel-denominated. The escape is raster-level, which favors physical, so the
-    // row count is scaled by the current RasterizationScale: at 100% the trigger is 10K (the
-    // measured-safe margin), at 200% it is 5K, at 300% ~3.3K — conservative under either
-    // hypothesis, while every realistic hosts file keeps the true-Mica look.
-    private const int StatusBackplateThreshold = 10_000;
-
-    public Visibility StatusBackplateVisibility =>
-        Entries.Count * (Content?.XamlRoot?.RasterizationScale ?? 1.0) > StatusBackplateThreshold
-            ? Visibility.Visible
-            : Visibility.Collapsed;
-
     // The status row is 0 while the archive view is shown: its content collapses via
     // MainViewVisibility, but a FIXED 32px grid row keeps its height regardless — which left a
     // permanent dead band of blank Mica under the archive panel.
@@ -531,10 +515,6 @@ private void UpdateStatusCounts()
             _statusText = text;
             OnPropertyChanged(nameof(StatusText));
         }
-
-        // Cheap: x:Bind just re-reads Entries.Count and the rasterization scale. Raised on every
-        // recount so the backplate tracks the visible row count through loads, filters, and bulk edits.
-        OnPropertyChanged(nameof(StatusBackplateVisibility));
     }
 
     // A filter change can swap the entire visible set, so rebuild it with a single bulk rebind
@@ -782,14 +762,7 @@ void ApplyMinimumWindowSize()
                         }
 
                         xamlRootChangedHooked = true;
-                        rootElement.XamlRoot.Changed += (_, _) =>
-                        {
-                            ApplyMinimumWindowSize();
-
-                            // The backplate trigger is scale-dependent too (raster-level escape —
-                            // see StatusBackplateVisibility).
-                            OnPropertyChanged(nameof(StatusBackplateVisibility));
-                        };
+                        rootElement.XamlRoot.Changed += (_, _) => ApplyMinimumWindowSize();
                     };
                 }
             }