fix(popup): add a workaround to make popup focusable despite being no…

…t IsTabStop

src/Uno.UI.RuntimeTests/MUX/Microsoft_UI_Xaml_Controls/SplitButton/SplitButtonTests_InteractionTests.cs

@@ -145,7 +145,8 @@ public async Task TouchTest()
 			var injector = InputInjector.TryCreate() ?? throw new InvalidOperationException("Failed to init the InputInjector");
 			using var finger = injector.GetFinger();
 
-			Log.Comment("Check simulate touch mode checkbox"); // Uno Doc: this is not needed, we use an injected touch pointer
+			// Uno Doc: this is not needed, we use an injected touch pointer
+			Log.Comment("Check simulate touch mode checkbox");
 			// simulateTouchCheckBox.Click(); // This conveniently moves the mouse over the checkbox so that it isn't over the split button yet
 			simulateTouchCheckBox.ProgrammaticClick();
 			await WindowHelper.WaitForIdle();
@@ -243,7 +244,7 @@ public async Task KeyboardTest()
 
 			Verify.AreEqual("0", flyoutOpenedCountTextBlock.Text);
 			Log.Comment("Verify that pressing alt-down on SplitButton opens the flyout");
-			KeyboardHelper.PressKeySequence("$d$_alt#$d$_down#$u$_down#alt");
+			KeyboardHelper.PressKeySequence("$d$_alt#$d$_down#$u$_down#$u$_alt");
 			await WindowHelper.WaitForIdle();
 			Verify.AreEqual("1", flyoutOpenedCountTextBlock.Text);
 

src/Uno.UI/UI/Xaml/Controls/Popup/Popup.cs

@@ -311,4 +311,7 @@ internal Brush LightDismissOverlayBackground
 
 	internal static DependencyProperty LightDismissOverlayBackgroundProperty { get; } =
 		DependencyProperty.Register(nameof(LightDismissOverlayBackground), typeof(Brush), typeof(Popup), new FrameworkPropertyMetadata(defaultValue: null, propertyChangedCallback: (o, e) => ((Popup)o).ApplyLightDismissOverlayMode()));
+
+	// On WinUi, a popup is not IsTabStop but is somehow focusable. This is a workaround to match that behaviour.
+	internal override bool IsFocusableForFocusEngagement() => true;
 }