I couldn't reproduce this, it probably depends on which fonts you have installed.

Do you still see the issue when logged out, or on another device?

Yes, I did actually experience this on multiple devices, from desktop computers to computer tablets.

Anyways, I suppose it should actually force the use of a (common) fixed-width monospace font instead of somehow using weird fonts that mess things up?

It should be using the same font as the old wikitext editor. You can set it in preferences: Preferences → Editing → Edit area font style on en.wp, 参数设置 → 编辑 → 编辑区字体样式 on zh.wp.

It is actually not using monospace fonts as set in my preferences.

So... are there any ways to solve this problem?

Btw, in enwp, it does correctly use monospace fonts so that isn't a problem, but this problem exists in zhwp where it's not using monospace fonts as configured.

Thread bump and update: it seems to using the very same font for any configurations of edit tool font set in my preferences (on zhwiki), and in fact has a different character spacing causing the shift. Any idea what could fix this?

@matmarex

@matmarex
I finally found out what exactly the problem is.
Using Traditional Chinese and Simplified Chinese as display language, the fonts used are also different.
When using Traditional Chinese (incl. Trad. Chinese (Taiwan), Trad. Chinese (Hong Kong), Trad. Chinese (Macao)) as display language on Chinese wikis, the editor first loads with the Simplified Chinese font by default and then the syntax highlight overlay uses the Traditional Chinese font. Since the two fonts have different character widths, the editor doesn't actually match with the overlay. I can also reproduce this even in Fandom which also supports syntax highlighting. Please check if you can reproduce this again using Traditional Chinese as the display language.