A blog by Devendra Tewari
The following code, added to the constructor of your main page, can help extend your views into the titlebar region
ApplicationViewTitleBar titleBar = ApplicationView.GetForCurrentView().TitleBar;
titleBar.ButtonBackgroundColor = Colors.Transparent;
titleBar.ButtonInactiveBackgroundColor = Colors.Transparent;
CoreApplicationViewTitleBar coreTitleBar = CoreApplication.GetCurrentView().TitleBar;
coreTitleBar.ExtendViewIntoTitleBar = true;
It is a slightly modified version of the original code available from StackOverflow. It makes the background of titlebar buttons transparent in the active state, and inactive state—when the window is out of focus. The titlebar region still allows you to use the mouse to drag the window around.
That may not be enough if you’re using a Microsoft.UI.Xaml.Controls.NavigationView control from Microsoft.UI.Xaml version 2.4.2. Although the menu itself extends into the titlebar, its content does not.
One workaround to make the content extend into the titlebar involves setting a negative top margin for the inner frame, like so
</winui:NavigationView.MenuItems>
<Frame x:Name="shellFrame" Margin="0, -32, 0, 0" />
</winui:NavigationView>
Another option is to set the IsTitleBarAutoPaddingEnabled property to False, but then the back button and hamburger menu button are overshadowed by the titlebar region, when they’re located there due to a smaller window size, and cannot be clicked.