Steps to replicate the issue (include links if applicable):
I'm not able to give a scenario which would always give the same results, but let's try:
- Go to this page (make sure the WikiEditor, not VE-like 2017 wikitext editor, is enabled)
- Click preview
What happens?:
The TOC doesn't load, the preview area is in the column aligned to the left, leaving a huge empty space on the right
What should have happened instead?:
The TOC should load, the preview area should be in the middle column
More info
I tried to replicate it with my staff account on Firefox, and I succeeded. Then I used my volunteer account on Chrome, and the problem didn't exist. With the staff account, I used safemode=1, and still had this issue. I tested against browsers (Firefox and Edge, staff: ❌, Firefox and Chrome, volunteer: ✅). So no relevance; looks like it's customization except safemode=1☝️. I checked my prefs and global prefs for wikitext editors (2017, 2010) and made them identical – no impact.