Bug title: WinUI 3 Popup SystemBackdrop bug #10677
Description
Describe the bug
WinUI 3 Popup SystemBackdrop bug
WinUI 3 Popup control Settings ShouldConstrainToRootBounds="False" and SystemBackdrop, behind of mica/acrylic background show not to come out, only the transparent color, I hope it can be fixed in the official version 1.8 of the Windows App SDK.
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WinUI 3 Popup 控件设置 ShouldConstrainToRootBounds="False" 和 SystemBackdrop 后，后面的云母/亚克力背景显示不出来，只有透明色，希望能在 Windows App SDK 1.8 正式版修复。
Steps to reproduce the bug
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Expected behavior
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Screenshots
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NuGet package version
WinUI 3 - Windows App SDK 1.7.3: 1.7.250606001
Windows version
Windows 11 (24H2): Build 26100
Additional context
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