List of steps to reproduce:

• Edit a page with WikiEditor (2010) and enable CodeMirror
• Scroll down with mouse wheel as much as you can. Note the empty blank space below last 32nd line.
• Click on noinclude at 26th line (e.g. between o and i letter). Note the caret position.
• Disable Syntax highlighting with pencil button. Then re-enable it clicking the pencil again. Note the scrollbar is not at bottom utmost.
• Scroll down.
• Click on 26th line. Note the caret position.

What happens?:
You may scroll down higher than last line.
The caret blinks at 24th line: your cursor is at 24th line whereas you click on 26th line.
After a while or some actions (e.g. scroll up and click several times in textearea), bottom blank space is removed and your click location becomes consistent with cursor location.

What should have happened instead?:
Where you click should always be where the cursor becomes located.

Software version, regression info:
Tested with Firefox 91 and Chromium 90.
I had not notice this really annoying bug before the previous week, that’s why I tag it as a regression.