Steps to replicate the issue:
- Open a discussion page, e.g. WP:VPT, on Firefox
- Double-click the first word in the first paragraph of a section
What happens?:
The heading gets selected in addition to the clicked word.
What should have happened instead?:
Only the word gets selected.
Other information:
The culprit appears to be this commit for T317135 (which ironically is about a Chromium bug). If you remove position: absolute; from span[data-mw-comment-start] (or set it to anything other than absolute or fixed), the bug goes away.