AssetCardList: Flash occurring in editor when switching focus between windows

[jfo8000]

https://github.com/microsoft/pxt/blob/d0ec5d92f120492142330e264cff57ba5d5cb364/webapp/src/components/assetEditor/assetCardList.tsx

The Asset Card List appears to be (causing or participating in?) a major flash of the editor whenever a window resize message is happening.

Recommend using react state variable and changing the state on the AssetEditor component instead of requesting an animation frame. This is more typically how you request a rerender?

I have tabbed away, then tabbed back and the thing running while the screen is black is this: