Bug: non-Hook startTransition waits for stale work to finish

[gaearon]

Not sure if this is a bug, but it breaks my expectations.

Here's the same example with useTransition and startTransition.

• useTransition: https://codesandbox.io/s/kgs8wi?file=/App.js
• startTransition: https://codesandbox.io/s/delicate-mountain-fr04u3?file=/App.js

Repro steps:

1. Click "Posts"
2. Immediately click "Contact"

With useTransition, the state change is instant because rendering slow Posts component is abandoned.

With startTransition, the state change is slow because it waits for the Posts component to finish rendering anyway.

[sophiebits]

Per @acdlite and @sebmarkbage:

• Any urgent update will interrupt the pending transition. This is why useTransition works, because under the hood it sets an isPending state variable outside the transition.
• This only affects CPU suspense; if the render is blocked on a loading promise then it should work fine.
• If the transitions are entangled it's a bug as pointed out here; if they're not then it's a bug that isPending interrupts it.

[billnye2]

Hello, any status update on if this is a feature or a bug? Just ran into this myself when debugging tearing with Jotai. startTransition caused tearing, useTransition fixed it. It was unexpected to me that startTransition and useTransition had different behavior. Thank you