Window.ExtendsContentIntoTitleBar can't be set in XAML

[niels9001]

Describe the bug

The Window class in WinUI 3 has a ExtendsContentIntoTitleBar property. However, it's not usable and requires a developer to set this in code-behind.

When set in XAML, the project does not compile throwing the following error: The XAML Binary Format (XBF) generator reported syntax error '0x09C4' : Property Not Found

THIS DOES NOT WORK

THIS WORKS

Steps to reproduce the bug

1. Create a new project
2. Open MainWindow.xaml
3. Set ExtendsContentTitleBar="True" in the Window object
4. Press F5
5. Error is thrown and does not compile.

Workaround

6. Open MainWindow.xaml.cs
7. Set ExtendsContentIntoTitleBar in the contructor.
8. Press F5

Expected behavior

This should just work in XAML as it's shown by IntelliSense and it's a property that can be set in code-behind.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.7 Preview 1: 1.7.250208002-preview1

Windows version

Windows 11 (24H2): Build 26100

Additional context

No response

[github-actions]

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Closed similar issues:

• Setting ExtendsContentIntoTitleBar in XAML causes WMC0612 (#9100), similarity score: 0.87
• Unable to set Window Title in XAML (#3689), similarity score: 0.83

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[DarranRowe]

Yes, there are long standing issues here where GenXbf.dll doesn't understand ExtendsContentIntoTitleBar, and there is also an issue with Title where the value for that differs.

I wonder if this is caused by the fact that GenXbf.dll would have naturally started its life off as targetting the UWP, and either the binary format generator or the binary format was never extended to understand additional properties on Window. Windows.UI.Xaml.Window has no ExtendsContentIntoTitleBar, this was a work around for WinUI 3.