Hi,
I noticed this bug a few months ago in Manjaro, and couldn't find the
surce of the bug. It happened both in Mousepad and Gedit (using XFCE).
I moved back to Debian testing (also XFCE) for that other reasons, and
didn't have that bug. Until this week.
The length of an 8-character tab is not always 8. Sometimes it's around
8.5 (visually), causing wrong results. Changing the length of the
indent (sometimes) fixes the problem, and sometimes it doesn't.
Using:
Mousepad 0.4.2
Debian Bullseye (11, testing)
Alex.