TitleBar - NRE when visibility is Collapsed

[luca-domenichini]

Describe the bug

When TitleBar is set to a Collapsed visilibity, the Window it resides in throws NRE continuously when hovering with mouse:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Wpf.Ui.Controls.TitleBar.HwndSourceHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) in C:\dev\wpfui-mimmo\src\Wpf.Ui\Controls\TitleBar\TitleBar.cs:line 679
   at System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

In this execution path we are trying to handle the hovering on buttons: if buttons are not yet built, we can safely skip this check.

I checked this issue too #962 but seems not related to this situation.

To Reproduce

1. Open WPF-UI solution, Wpf.Ui.Gallery project.
2. Edit Wpf.Ui.Gallery.Views.Windows.MainWindow by adding Visibility="Collapsed" to TitleBar, as in following screenshot:

3. Set breakpoint at TitleBar line ~678
4. Run Gallery program and move mouse inside window (bug does not happen when mouse is not hovering Window)
5. Breakpoint hits with NULL values inside the _buttons array:

The problem is that _buttons array is filled with buttons in OnApplyTemplate() method, that is not called if TitleBar is collapsed.

Expected behavior

The expected behavior is to simply fallback to no-op on buttons hovering check.

Screenshots

No response

OS version

Win11

.NET version

.NET 10

WPF-UI NuGet version

4.2.0

Additional context

No response

[chucker]

What is the use case for collapsing a title bar?

[luca-domenichini]

I am using FluentWindow + TitleBar within a base class with cs + xaml + a dependency property ShowTitleBar bound to TitleBar visibility. This way I can reuse the same base window class and adjust the visibility of titlebar based on specific needs.

Offcourse, I could make 2 base classes instead of one, or make 1 base class without titlebar and another one that inherits on the former and just adds it, but, anyway, I think this is a bug that should be addressed since TitleBar is throwing NRE badly with a very basic valid value for Visibility.