Changes are lost when collapsing a paragraphs subform including an inline_entity_form

Tested your patch, and it does work on simple widget but I think we should implement this on complex widget too. Also small coding standards issues:

1. Expected one space after closing parentheses.

+    if (!$widget_state){
+      $form_state->set(['inline_entity_form', $ief_id], []);
+    }

2. I think moving the comment from EntityInlineForm to InlineFormInterface should be a separated issue(or added to existing similar issue) since it's not related to this one and should be remove from the patch.

I just tested the patch. As @nkoporec mentions, it works for simple widget but not for complex widget. For complex widget, there is an Update button though so it could be a workaround. But I do agree that it needs to be implemented with complex widget as well.

I am having an issue using complex IEF that are nested in a node. I have a 1:1 product and variation in an inline entity form, and when you attempt to edit the form, you can update the product (parent), but but you cant update the child (variation). If you save the node with the IEF open in edit mode, the child updates as you would expect it to by simply updating the IEF.

It sounds like this is a common problem, unrelated to products from this on Drupal Answers, so I am wondering if this is the right issue?

patch rerolled

forgot to add the import

I think it would really help to have tests here to avoid breaking this again in future.

IEF rc9 is out and has some fixes for nested forms. Can this still be reproduced?

The problem continues and it cannot be applied

Reroll of #12

I tried to build a small test the shows that there is a general problem during ajax requests, that the form is not rebuild properly.

The test is quite nice, I think. And the actual patch has proven to fix the problem for years now and we need it for multiple modules. "Entity reference actions" and "Media Library Media Modify" both have it as patches in their composer.json files.
Would be really nice to get this one in!

TBH i do not completely grok that, but changes look reasonable, and we now have a test.
If someone makes a MR, it's easier for me to commit.

Sure, here you go :)

Thanks everyone for working on this!
Committed and rolled new release. Rock on!

This commit caused regressions with non paragraph IEF fields. I have an (albiet complex) IEF implementation for user that completely broke after this commit. Any chance this could see a revert while a test coverage and fixes are added?

Opened #3270348: Regression from #2901158.

As @heddn noted, this change caused a regression, so it was reverted in #3270348: Regression from #2901158.
Please continue in #3271293: Changes are lost when collapsing a paragraphs subform including an inline_entity_form