FindChild Doesn't Search Expander Header

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Describe the bug

The FindChild methods in FrameworkElementExtensions fail to search for FrameworkElements in the Header of an Expander control. A Framework control is treated as a ContentControl, but the library only recursively searches the Content of the ContentControl, but an ExpanderControl also has an additional Header that gets skipped.

Steps to reproduce

Repro steps:
1. Create an expander control with Header & Content.
2. Put a demo control in the Header to search for.
3. Write a test using VisualUITestBase that loads the control.
4. Attempt to search the Expander control for a sub control under the Header.
5. The search should return an empty result.

Expected behavior

I would expect to be able to find FrameworkElements in the Header of an Expander control using the FindChild methods in FrameworkElementExtensions.

Screenshots

No response

Code Platform

• UWP
• WinAppSDK / WinUI 3
• Web Assembly (WASM)
• Android
• iOS
• MacOS
• Linux / GTK

Windows Build Number

• Windows 10 1809 (Build 17763)
• Windows 10 1903 (Build 18362)
• Windows 10 1909 (Build 18363)
• Windows 10 2004 (Build 19041)
• Windows 10 20H2 (Build 19042)
• Windows 10 21H1 (Build 19043)
• Windows 10 21H2 (Build 19044)
• Windows 10 22H2 (Build 19045)
• Windows 11 21H2 (Build 22000)
• Other (specify)

Other Windows Build number

No response

App minimum and target SDK version

• Windows 10, version 1809 (Build 17763)
• Windows 10, version 1903 (Build 18362)
• Windows 10, version 1909 (Build 18363)
• Windows 10, version 2004 (Build 19041)
• Windows 10, version 2104 (Build 20348)
• Windows 11, version 22H2 (Build 22000)
• Other (specify)

Other SDK version

No response

Visual Studio Version

2022

Visual Studio Build Number

No response

Device form factor

No response

Additional context

No response

Help us help you

Yes, I'd like to be assigned to work on this item.

[Arlodotexe]

Expander is coming from the platform, so this is something we should be able to test for once we fix #474.