Render the vertical tabs widget server side to eliminate the flash of untransformed content

Lauri Timmaneerequested to merge
issue/drupal-3615107:3615107-render-the-vertical into main

Vertical tabs paint the server-side markup first - a stack of expanded details elements - and only become the tabs widget once footer JavaScript runs. On /admin/structure/types/manage/article the stack is visible for 2-5 frames per load before it snaps into the widget.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

Closes #3615107

Merge request reports