Embedded videos flicker

I was asked to move this here from #1096.

Description

Here's what I see:

https://user-images.githubusercontent.com/606923/157578617-d64463c7-b5ae-4737-940c-07d6677448a8.mp4

Here's what I figured out:

If I remove the CSS property video { border-radius: 8px; } the flickering goes away.

I tried creating a sample project but I'm having trouble getting it to reproduce. It seems to be dependent on the HTML/CSS plus the context of the Xaml containing the WebView2. In a new WinUI 3 project with a similarish XAML layout and similarish CSS rules, I cannot get any (or the same) video to flicker (even with border-radius set) and I can't share the app I have where it does.

Version
SDK: WindowsAppSdk 1.0
Runtime: "5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36 Edg/99.0.1150.36"
Framework: Win32 WinUI3
OS: Windows 10 18363

Repro Steps
Unknown. It seems border-radius is a precondition for this to happen, but not sufficient on its own. Does not readily reproduce in Edgeium.

Screenshots
See above.

Additional context
Did not use to happen, so it's some sort of regression in WinUI 3 (how the WebView2 is hosted/embedded) or WebView2 itself.

AB#38484159

Start with the attached video and the stated WebView2/WinUI 3 versions; compare behavior with and without video { border-radius: 8px; } in the affected host. Done requires a reliable reproduction and a confirmed fix or documented component ownership, since the current report says repro steps are unknown.