useResizeObserver: handle strict mode double effects

[jsnajdr]

Fixes useResizeObserver in strict mode. Which unmounts and remounts effects twice, and the second mount didn't update didUnmount.current back to false, so the hook was considered permanently unmounted.

[ntsekouras]

I tested only in local dev mode for now.

1. The first issues I observed with site editor and previews seem fine
2. The second issue with history(undo/redo) in post editor persists.

Screen.Recording.2023-02-02.at.7.42.53.PM.mov

There might be more issues though.. I haven't tested thoroughly..

[ntsekouras]

Also saw this one:

[ciampo]

I confirm what @ntsekouras says — the site editor is back, but sometimes the undo button does not activate, and the "restore" banner appears twice.

As Nik mentions, there could likely be more issues that we may find by testing this PR.

Maybe the better idea is to revert the offending PR, and work on a fix with less urgency? I'm just afraid that we'd introduce more regressions without careful testing

[ciampo]

We can probably merge this hotfix while still reverting the original Strict Mode PR

[WunderBart]

Do we need to add regression tests for this? I guess it should have been caught in the original PR (where we enabled the Strict Mode).

[jsnajdr]

Do we need to add regression tests for this?

To catch this, we'd have to run the e2e tests against a development build. In production mode, strict mode doesn't do anything. Only in dev it does things like re-mounting effects, exposing bugs like this one.

[jsnajdr]

but sometimes the undo button does not activate

There is some bug in the useBlockSync hook. When a block changes in core/block-editor, this hook should sync the change to a core entity, calling editEntityRecord. And it's the core store that maintains the undo/redo stack. Sometimes, this sync doesn't happen. Going to investigate this. useBlockSync is quite complex.