Proposal: CompositionBackdropBrush sampling over SwapChainPanel

Proposal: CompositionBackdropBrush sampling over SwapChainPanel

Follow up from https://github.com/microsoft/microsoft-ui-xaml/issues/4947

cc. @marb2000

Summary

As described in the related issue, currently CompositionBackdropBrush cannot sample from over a SwapChainPanel. This makes it impossible to apply effects such as GaussianBlurEffect over content being rendered in a SwapChainPanel. This proposal is about allowing content in a SwapChainPanel to be sampled just like other normal WinUI content in a window, so that it can be used as input in any composition pipeline that starts with a CompositionBackdropBrush.

Rationale

• The SwapChainPanel isn’t always used as a full-window renderer, but it’s often used to compose UI components in an application (eg. a map control, some custom renderer inside a template, etc.). With the current limitation in WinUI 3, this makes it impossible to then apply effects on top of the entire UI (eg. a blur effect if a popup is opened), because those SwapChainPanel-s would then visibly be “glitched” next to the normal WinUI 3 content behind the effect.
• Make the behavior consistent with UWP (this could be seen as a regression in WinUI 3)
• Make the behavior more intuitive for developers (“why are these effects not working here?”)

Scope

Capability	Priority
Allow CompositionBackdropBrush to sample over SwapChainPanel too	Must

[kevinguo305]

Thanks for submitting this proposal Sergio! It’s in our backlog and we’ll consider this for a future release.

[Sergio0694]

Bumping this (see https://github.com/microsoft/microsoft-ui-xaml/discussions/8638).