Revert "Revert "[Windows] Fix Flyout/Locked mode header collapse regression causing UI test failures on candidate branch""

src/Core/src/Platform/Windows/RootNavigationView.cs

@@ -68,6 +68,17 @@ internal Size FlyoutPaneSize
 
 		void PaneDisplayModeChanged(DependencyObject sender, DependencyProperty dp)
 		{
+			// AlwaysShowHeader must be false only in LeftMinimal mode (Shell flyout) so that
+			// CollapseEmptyHeader() can remove the header space when title/items are empty.
+			// All other modes (e.g. Left / FlyoutBehavior.Locked) keep AlwaysShowHeader=true
+			// so the header area remains visible regardless of content — restoring the
+			// original NavigationView behaviour and preventing the Issue2740 regression.
+			// Note: Top mode (TabbedPage) is safe with AlwaysShowHeader=true because
+			// UpdateToolbarPlacement() sets Header=null there, and WinUI's HeaderContent
+			// (a ContentControl) auto-collapses to zero height when its content is null.
+			// Additionally, UpdateHeaderPropertyBinding() binds HeaderContent.Visibility
+			// directly to Toolbar.Visibility, bypassing AlwaysShowHeader entirely.
+			AlwaysShowHeader = PaneDisplayMode != NavigationViewPaneDisplayMode.LeftMinimal;
 			UpdateToolbarPlacement();
 			UpdatePaneContentGridMargin();
 			UpdateHeaderVisibility();