Fix missing backgrounds in RecentPinnedLocations and GroupBox (#23)

* Fix missing backgrounds in RecentPinnedLocations and GroupBox

- RecentPinnedLocations template was missing a Background binding on its
  Grid, unlike all sibling container templates (ArrayPanel, Container, etc.).
  Added the same StringToBrushConverter binding.

- GroupBox content-area background used FallbackValue=#66212121, which only
  fires on binding errors. When StringToBrushConverter returns null (empty
  string or null Background value), Avalonia sets the property to null
  (transparent). Changed to TargetNullValue so the default dark fill is
  applied whenever the converter produces null.

Fixes #16

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Fix missing backgrounds in RecentPinnedLocations and GroupBox

- RecentPinnedLocations Grid was missing a Background binding; added
  StringToBrushConverter binding matching all other container templates
- GroupBox content area used FallbackValue for null Background; changed
  to TargetNullValue so the default #66212121 fill applies when the
  converter returns null (not just on binding errors)
- ScrollPanel content now has MinHeight bound to the ScrollViewer's
  viewport height so DockPanel LastChildFill works correctly: Avalonia's
  ScrollViewer passes infinite height to content, collapsing the GroupBox
  to zero height when empty; the MinHeight constraint gives the DockPanel
  a finite lower bound while still allowing overflow scrolling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Fix ScrollPanel content not filling viewport for DockPanel LastChildFill

The previous fix (MinHeight only) was insufficient: the default StackPanel
items panel gives each child only its desired height regardless of MinHeight,
so the inner DockPanel never received the viewport height as a finite
constraint and LastChildFill could not allocate remaining space to the
Pinned Locations GroupBox.

Fix: switch the ScrollPanel items panel from StackPanel to Grid. A Grid
stretches its single content child to fill the full arranged height, so
the DockPanel receives a finite height equal to the viewport and
LastChildFill correctly allocates remaining space to the last child.
MinHeight is retained to ensure the Grid's arranged height is at least
the ScrollViewer's visible viewport height even when content is shorter,
while content taller than the viewport still overflows and scrolls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: EmoSaru <emosaru@emosaru.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

EmoTracker/UI/LayoutControl.axaml

@@ -56,6 +56,7 @@
                 </LayoutTransformControl.LayoutTransform>
                 <Grid Width="{Binding Width, Converter={x:Static converters:NegativeToNaNDoubleConverter.Instance}}"
                       Height="{Binding Height, Converter={x:Static converters:NegativeToNaNDoubleConverter.Instance}}"
+                      Background="{Binding Background, Converter={x:Static converters:StringToBrushConverter.Instance}}"
                       IsHitTestVisible="{Binding HitTestVisible}"
                       MinWidth="{Binding MinWidth, Converter={x:Static converters:NegativeToZeroDoubleConverter.Instance}}"
                       MinHeight="{Binding MinHeight, Converter={x:Static converters:NegativeToZeroDoubleConverter.Instance}}"
@@ -372,7 +373,7 @@
                     </Grid>
                     <!-- Content area -->
                     <Grid Grid.Row="1"
-                          Background="{Binding Background, Converter={x:Static converters:StringToBrushConverter.Instance}, FallbackValue=#66212121}">
+                          Background="{Binding Background, Converter={x:Static converters:StringToBrushConverter.Instance}, TargetNullValue=#66212121, FallbackValue=#66212121}">
                         <ItemsControl ItemsSource="{Binding Items}" Margin="5"
                                       ItemContainerTheme="{StaticResource StretchItemContainer}">
                             <ItemsControl.ItemsPanel>
@@ -409,7 +410,20 @@
                       Effect="{Binding DropShadow, Converter={x:Static converters:BoolToDropShadowEffectConverter.Instance}}">
                     <ScrollViewer HorizontalScrollBarVisibility="{Binding HorizontalScrollBarVisibility, Converter={x:Static converters:TrivialEnumConverter.Instance}}"
                                   VerticalScrollBarVisibility="{Binding VerticalScrollBarVisibility, Converter={x:Static converters:TrivialEnumConverter.Instance}}">
-                        <ItemsControl ItemsSource="{Binding Items}" />
+                        <!-- Use a Grid panel (not the default StackPanel) so the single content
+                             child is stretched to fill the ItemsControl's full arranged height.
+                             MinHeight ties that height to the ScrollViewer's visible viewport so
+                             DockPanel LastChildFill receives a finite height and allocates the
+                             remaining space to the last child (e.g. the Pinned Locations GroupBox).
+                             Content taller than the viewport still overflows and scrolls normally. -->
+                        <ItemsControl ItemsSource="{Binding Items}"
+                                      MinHeight="{Binding $parent[ScrollViewer].Bounds.Height}">
+                            <ItemsControl.ItemsPanel>
+                                <ItemsPanelTemplate>
+                                    <Grid />
+                                </ItemsPanelTemplate>
+                            </ItemsControl.ItemsPanel>
+                        </ItemsControl>
                     </ScrollViewer>
                 </Grid>
             </LayoutTransformControl>