Avoid manipulating PackedScene cache when generating scene preview thumbnails

[daniel080400]

Attempt to fix issue #107494

Should likely be a cache issue with ResourceLoader. Need further testing and MRP.

Edit:
Issue confirmed is related to cache mode, see comments below.

• Bugsquad edit, fixes: Scripts detached from child-scenes in the editor when the scene is loaded. #107494

[daniel080400]

I confirmed the issue is fixed by changing the cache mode from CACHE_MODE_IGNORE to CACHE_MODE_IGNORE_DEEP, using the steps to reproduce mentioned in #107494 with an blank project created after 4.4.1. (Previously I tested with complex projects that are created prior to 4.4.1 and it's somehow not reproducible)

Open any scene containing child scenes with script. Change something. Don't save. Reload the scene via Scene -> Reload Saved Scene -> Save & Reload. All child scenes have 'lost' their script.

With the same project, the issue persists when changing it back to CACHE_MODE_IGNORE, and fixed when changing it to CACHE_MODE_IGNORE_DEEP.

Marking this as ready for review, this change should have no side effects, we should merge it right away. May someone provide more testing results to let us confidently mark the issue as resolved.

[pmoosi]

Can confirm that it fixes the issue for me.

[KoBeWi]

Fixes the issue for me too.

[poglad-ds]

After migrating from 51b0379 (11.06 build date) that have this problem, this looks like fixes it as long as i see.

[akien-mga]

Thanks!