Revert "Migrate MR rapid diffs page to Vue 3"

Stanislav Lashmanovrequested to merge
slashmanov/rapid-diffs-revert-vue3-migration into master

What does this MR do and why?

Reverts !252389 (merged) ("Migrate MR rapid diffs page to Vue 3").

With vue3_migrate_mr_rapid_diffs on, replying to a diff discussion on the Rapid Diffs merge request page crashes under @vue/compat. The reply is saved but never renders: the optimistic placeholder note carries no author, and Rapid Diffs has no isPlaceholderNote branch of its own (legacy notes/components/discussion_notes.vue does), so it renders the placeholder as a regular note and throws TypeError: Cannot read properties of undefined (reading 'id'), followed by Cannot set properties of null (setting '__vnode') in Vue's patchElement.

The flag is default_enabled: false, so GitLab.com and self-managed are unaffected. CI is: unpersisted feature flags are enabled by default in RSpec, so every feature spec serves the Vue 3 entrypoint. Three examples fail — two in spec/features/merge_request/user_comments_on_whitespace_hidden_diff_spec.rb, one in spec/features/merge_request/user_sees_avatar_on_diff_notes_spec.rb.

Let's fix the placeholder-note gap in Rapid Diffs first, then re-apply the migration. Rollout work item: #611465.

Reproduction steps

  1. Enable the vue3_migrate_mr_rapid_diffs feature flag.
  2. Open a merge request's Changes tab.
  3. Comment on a diff line.
  4. Reply to the thread you just created.
  5. The reply never appears, and the console shows the TypeError.

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