## Summary

Primer Begin of the insertionTreeView'sEnd of the insertion Begin of the deletion TreeView'sEnd of the deletion form integration (`form_arguments:`) maintains hidden inputs for checked tree items via a MutationObserver. The Begin of the insertionobserver'sEnd of the insertion Begin of the deletion observer'sEnd of the deletion guard only recognized `aria-checked` attribute changes, so checked nodes **inserted** after the component connects (e.g. async fragments) appeared selected in the tree but were omitted from form submission. Code review also confirmed the mirror gap: checked nodes **removed** from the DOM leave a stale hidden input behind.

## Scope

Begin of the insertion* End of the insertion Begin of the deletion -End of the deletion Insertion gap: fixed in Begin of the insertionhttps://github.com/opf/primer\_view\_components/pull/515End of the insertion Begin of the deletion https://github.com/opf/primer_view_components/pull/515End of the deletion (follow-up to Begin of the insertionhttps://github.com/opf/primer\_view\_components/pull/512)

* End of the insertion Begin of the deletion https://github.com/opf/primer_view_components/pull/512)
-End of the deletion Removal gap + `attributeOldValue` observer hardening: to be added to the same PR Begin of the insertion

* End of the insertion Begin of the deletion
-End of the deletion ESLint cleanup (7 pre-existing errors) + changeset, same branch Begin of the insertion


End of the insertion Begin of the deletion

End of the deletion ## Relation to OP-19878

https://community.openproject.org/wp/OP-19878 / https://github.com/opf/openproject/pull/24757: the projects-tab Stimulus controller re-implements TreeView form sync app-side with its own MutationObserver and JSON hidden input. Once this fix (incl. removal handling) ships in the gem, that controller can be replaced by Begin of the insertionTreeView'sEnd of the insertion Begin of the deletion TreeView'sEnd of the deletion native `form_arguments:` hidden inputs. Begin of the insertion

### Hints for QA

This is logged as a code correctness bug. This issue currently manifests itself as the user-facing bug <mention class="mention" data-id="78282" data-type="work_package" data-text="#OP-19878" data-display-id="OP-19878">#OP-19878</mention> (although that will received a separate, interim fix)End of the insertion