Bug: Idle updates may cause normal priority updates to get "stuck"

[gaearon]

This repro is quite convoluted but it's the best I could extract. It seems like numerous things need to happen to trigger this bug. To reproduce:

1. Open https://codesandbox.io/s/great-wildflower-mv89b and wait a second
2. Observe that there is only one "Hello" (for UB). Normal update got stuck.
3. Also notice how clicking "wake up" (which schedules a dummy update) will let the other updates "come through".

You may need to tweak the timing of CPUSpin component if this doesn't repro for you.

[gaearon]

This appears to have been fixed — possibly, by Lanes.
https://codesandbox.io/s/friendly-browser-3hhh0

I don't know how to write a test for it so I'll just close.