[IMPROVEMENT] Restore Mica Fade-out When Losing Focus

[Meteony]

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This improvement refers to an existing feature. If you want to suggest a new feature, please use this template.
• This improvement is not a bug. If you want to report a bug, please use this template.

Describe the improvement

WinUI turns off Mica for windows that aren't in focus to both visually distinguish those windows that aren't in focus and conserve device power. This feature is missing from the Avalonia rewrite.

UniGetUI_.2026-07-18_20.20.32.mp4

UniGetUI_.2026-07-18_20.22.20.mp4

Describe how this improvement could help users

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