Fix TabView tear-out sample: close empty windows and clear stale refs

[niels9001]

Description

Note

This PR was generated with the assistance of GitHub Copilot (AI). All changes should be reviewed carefully.

Fixes #2078

The TabView windowing sample (TabViewWindowingSamplePage.xaml.cs) has issues where empty windows are left open after tab tear-out/drop-back operations, and stale window references can cause unexpected behavior across multiple tear-out sequences.

Root Cause Analysis

Issue #2078 reports three problems:

1. Screen flickers to black when clicking tabs — this is a WinUI framework behavior where TabTearOutWindowRequested fires on mouse-down rather than on drag gesture. This is not fixable in the sample code (framework-level issue).
2. App crashes after repeated tab switching — partially caused by accumulated empty windows and stale tabTearOutWindow references.
3. No cleanup for empty windows after tabs are dropped back — the sample only closed windows via the tab close button, not when all tabs were torn out or dropped into another window.

Changes (1 file)

TabViewWindowingSamplePage.xaml.cs

Change	Why
Added CloseWindowIfEmpty(TabView) helper	Walks the visual tree from a TabView to find its parent Page, then closes the containing Window when the TabView is empty. Consolidates the pattern used across multiple handlers.
Close source window in Tabs_TabTearOutRequested	When all tabs are torn out to a new window, the source window was left open and empty. Now it closes automatically.
Close source window in Tabs_ExternalTornOutTabsDropped	When all tabs are dragged back into another window, the now-empty source window was left open. Now it closes automatically.
Clear tabTearOutWindow after tear-out completes	Prevents stale reference when multiple tear-outs happen in sequence. Each TabTearOutWindowRequested sets the field, but it was never cleared — so Tabs_TabTearOutRequested could reference a wrong window.
Refactored Tabs_TabCloseRequested	Now uses the same CloseWindowIfEmpty helper for consistency.
Removed unused DataIdentifier constant	Dead code cleanup.

What this does NOT fix

The screen flicker on tab click (issue point #1) is caused by TabTearOutWindowRequested firing on mouse-down rather than on an actual drag gesture. This is a WinUI framework behavior, not a sample code issue. A fix would need to come from the microsoft-ui-xaml repo.

How to validate

1. Build: dotnet build WinUIGallery\WinUIGallery.csproj /p:Platform=x64 — 0 errors ✅
2. Manual testing (requires running the app):
   • Open the TabView page → click "Click here to launch the sample"
   • Test tear-out cleanup: Drag all tabs from the main sample window to a new window → verify the source window closes automatically
   • Test drop-back cleanup: Tear out a tab to a new window → drag it back to the original window → verify the now-empty window closes automatically
   • Test close button: Close all tabs via the X button → verify the window closes (existing behavior preserved)
   • Test multiple tear-outs: Tear out tabs multiple times in sequence → verify each operation creates a proper new window without interference from previous tear-outs