Spellchecking is important as per https://www.whonix.org/wiki/Surfing_Posting_Blogging.
I don’t like mousepad too because you can’t even edit font size by mouse wheel.
gedit has the same issue but at least font size can be increased using menu. Issue with gedit is you need to remember to do spellchecking manually by clicking the tool from the hamburger menu, bad usability.
Best is to show wrongly written words right when typing. That’s best chance to still remember context and correct (or keep if false positive) them.
kate still has the bug of showing sometimes words marked as spelling mistake even though they are not.
A high usability editor would be desirable.