Fix titlebar color when using backdrop in WPF Window

[pomianowski]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

Window can have a color in custom titlebar when Show accent colors option is enabled in Windows.

Issue Number: N/A

What is the new behavior?

After enabling Mica backdrop, the titlebar color should be removed.

[pomianowski]

FYI: @Jay-o-Way, @aquinn39
rel.: microsoft/PowerToys#30206

[jaimecbernardo]

@pomianowski Awesome work here :)
I've added this fix to microsoft/PowerToys#33458 , while 4.0.0 is not released.
Thank you!

Quick question: Why uint titlebarPvAttribute = 0xFFFFFFFE; and not uint titlebarPvAttribute = 0xFFFFFFFF; ?

[aquinn39]

Quick question: Why uint titlebarPvAttribute = 0xFFFFFFFE; and not uint titlebarPvAttribute = 0xFFFFFFFF; ?

I got the value (DWMWA_COLOR_NONE) from DWMWA_BORDER_COLOR here. This value appearantly hides the window border, so I thought maybe it might work for DWMWA_CAPTION_COLOR as well, which it did. 0xFFFFFFFF (DWMWA_COLOR_DEFAULT) just resets the title bar colour to the default system behaviour, which means using the accent colour if you have that option enabled - that behaviour actually is documented under DWMWA_CAPTION_COLOR.

Makes me wonder if DWMWA_COLOR_NONE (0xFFFFFFFE) also works on DWMWA_TEXT_COLOR to hide the caption text (not that it's necessary in this case).

[aquinn39]

Interestingly this behaviour is actually correctly documented in dwmapi.h.