Bug: Nested useTransition makes isPending of outer one always false

[gaearon]

Nested startTransition call "takes over" and makes parent startTransition unable to track isPending.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. isPending in IndexPage.js is true

However, isPending in App.js is false.

Expected: isPending in App.js is also true.