Mica BackDropMaterial not updating when RequestedTheme changes

Describe the bug The Mica material is not toggling between light and dark theme when RequestedTheme of the Page to which it is applied changes.

Steps to reproduce the bug

1. Apply Mica to Page (muxc:BackdropMaterial.ApplyToRootOrPageBackground="True")
2. Change the RequestedTheme Property of Page from Light to Dark

Expected behavior Background changes according to the theme

Screenshots

Version Info

NuGet package version: 2.7.0-prerelease.210827001

Windows app type:

UWP	Win32
Yes

Windows version	Saw the problem?
Insider Build (22454.1000)	Yes
May 2021 Update (19043)
October 2020 Update (19042)
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context

[eugenegff]

I had similar issue when I set muxc:BackdropMaterial.ApplyToRootOrPageBackground=“True” for control that then became Window.Content. The problem was that Mica infrastructure subscribed to the Window.Content.ActualThemeChanged event for soon-to-be-replaced Window.Content, and then lost the ability to feel theme changes.

Either do not change Window.Content, or set muxc:BackdropMaterial.ApplyToRootOrPageBackground=“True” in code after completing such change.

[jevansaks]

@eugenegff is likely correct. Unfortunately we don’t have a way to do RequestedTheme for a XamlRoot so you set it on the content but then if the content changes or if you set it on a child of the thing that is the root, we don’t know about it.

So you need to set it on Window.Current.Content.RequestedTheme and then don’t change Window.Current.Content.