CmdPal: Hide search box on content pages [experiment] (#41479)

## Summary of the Pull Request

CmdPal now displays the search box only on pages that derive from
ListPage. On ContentPage (forms, etc.), the search box is hidden.

- Moves keyboard shortcut handling from SearchBox to ShellPage so
shortcuts are always handled.
- Keeps the search box hidden/disabled to preserve layout metrics. 
- Refines focus management to prevent focus jumps during navigation. 
- For ContentPage page's content gains focus automatically (not just
form inputs, but now markdown content as well - so you can scroll
immediately, for example).
- Adds accessibility (a11y) tweaks: when navigating to content pages
without a visible search box, sets an explicit focus target so screen
readers announce a meaningful element. Screen reader will now announce
navigation to the page.
- Adds a title to the main list page - "Home".


https://github.com/user-attachments/assets/f60d0826-df1f-468e-8e41-0266cd27878b


<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #38967
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml

@@ -9,6 +9,7 @@
     xmlns:coreViewModels="using:Microsoft.CmdPal.Core.ViewModels"
     xmlns:cpcontrols="using:Microsoft.CmdPal.UI.Controls"
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:h="using:Microsoft.CmdPal.UI.Helpers"
     xmlns:help="using:Microsoft.CmdPal.UI.Helpers"
     xmlns:labToolkit="using:CommunityToolkit.Labs.WinUI.MarkdownTextBlock"
     xmlns:markdownImageProviders="using:Microsoft.CmdPal.UI.Helpers.MarkdownImageProviders"
@@ -16,7 +17,6 @@
     xmlns:tkcontrols="using:CommunityToolkit.WinUI.Controls"
     xmlns:toolkit="using:CommunityToolkit.WinUI.Controls"
     xmlns:ui="using:CommunityToolkit.WinUI"
-    xmlns:viewModels="using:Microsoft.CmdPal.UI.ViewModels"
     Background="Transparent"
     mc:Ignorable="d">
 
@@ -27,8 +27,6 @@
                 EmptyValue="Collapsed"
                 NotEmptyValue="Visible" />
 
-            <converters:BoolNegationConverter x:Key="BoolNegationConverter" />
-
             <cmdpalUI:MessageStateToSeverityConverter x:Key="MessageStateToSeverityConverter" />
 
             <cmdpalUI:DetailsDataTemplateSelector
@@ -175,9 +173,12 @@
 
             <!--  Back button and search box  -->
             <Grid
+                x:Name="TopBarGrid"
                 Padding="0,12,0,12"
                 HorizontalAlignment="Stretch"
-                VerticalAlignment="Stretch">
+                VerticalAlignment="Stretch"
+                BorderBrush="{ThemeResource DividerStrokeColorDefaultBrush}"
+                BorderThickness="0,0,0,1">
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="Auto" />
                     <ColumnDefinition Width="*" />
@@ -354,11 +355,7 @@
                 </animations:Implicit.HideAnimations>
             </ProgressBar>
 
-            <Grid
-                x:Name="ContentGrid"
-                Grid.Row="1"
-                BorderBrush="{ThemeResource DividerStrokeColorDefaultBrush}"
-                BorderThickness="0,1,0,1">
+            <Grid x:Name="ContentGrid" Grid.Row="1">
 
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="3*" />
@@ -367,6 +364,9 @@
 
                 <Frame
                     Name="RootFrame"
+                    AutomationProperties.AutomationControlType="Pane"
+                    AutomationProperties.LandmarkType="Navigation"
+                    AutomationProperties.LiveSetting="Assertive"
                     IsNavigationStackEnabled="True"
                     Navigated="RootFrame_Navigated" />
 
@@ -502,7 +502,11 @@
             </InfoBar>
         </StackPanel>
 
-        <Grid Grid.Row="1" Background="{ThemeResource LayerOnAcrylicSecondaryBackgroundBrush}">
+        <Grid
+            Grid.Row="1"
+            Background="{ThemeResource LayerOnAcrylicSecondaryBackgroundBrush}"
+            BorderBrush="{ThemeResource DividerStrokeColorDefaultBrush}"
+            BorderThickness="0,1,0,0">
             <cpcontrols:CommandBar CurrentPageViewModel="{x:Bind ViewModel.CurrentPage, Mode=OneWay}" />
         </Grid>
 
@@ -519,6 +523,26 @@
                     </VisualState.Setters>
                 </VisualState>
             </VisualStateGroup>
+            <VisualStateGroup>
+                <!--
+                    Why disable it and make it transparent?
+                    To prevent the container from collapsing, ensuring the layout metrics remain unchanged.
+                -->
+                <VisualStateGroup.Transitions>
+                    <VisualTransition GeneratedDuration="0:0:0.187" To="SearchBarCollapsed" />
+                </VisualStateGroup.Transitions>
+                <VisualState x:Name="SearchBarVisible" />
+                <VisualState x:Name="SearchBarCollapsed">
+                    <VisualState.StateTriggers>
+                        <StateTrigger IsActive="{x:Bind h:BindTransformers.Negate(ViewModel.IsSearchBoxVisible), Mode=OneWay}" />
+                    </VisualState.StateTriggers>
+                    <VisualState.Setters>
+                        <Setter Target="SearchBox.IsEnabled" Value="False" />
+                        <Setter Target="SearchBox.Opacity" Value="0" />
+                        <Setter Target="TopBarGrid.BorderBrush" Value="Transparent" />
+                    </VisualState.Setters>
+                </VisualState>
+            </VisualStateGroup>
         </VisualStateManager.VisualStateGroups>
     </Grid>
 </Page>