"The issue only occurs when making the width of the window something that isn't a full integer multiple of the monospaced text. In such a case, the internal screen moves to show the remainder by moving to the right."

I can't actually trigger this myself thus far.

Good catch on CodeMirror being a factor; I didn't notice it was enabled in the video. With that turned on I can reproduce this. It's something to do with the direct-child of the .CodeMirror-wrap div getting shifted slightly to the left when the caret is at the far-right side of the text.

Here's a video with some relevant tools shown: https://youtu.be/0yfp13Thgnc

It's probably worth starting out by seeing whether updating the CodeMirror library would fix this. It looks like we've got 5.58.3 (2020-11-19) bundled in the extension, and 5.65.13 (2022-12-20) is the most recent version, so there's ~2 years of fixes to catch up on. (Or there's the v6 version of the library, but I assume that'd be a riskier upgrade...)

(Or there's the v6 version of the library, but I assume that'd be a riskier upgrade...)

Risky indeed! I'd estimate it as at least a one-month project, if I were to tackle it on my own. The API is dramatically different so we need to rewrite a lot of things. The tracking task is at T259059: Upgrade to CodeMirror 6. Community Tech plans to work on this in 2023 but we don't have a set timeline yet. It's probably worth it to upgrade to the latest CodeMirror 5 now, if we want to. T258999 was the last upgrade.

Is this bug reproducible in the 2010 editor? If not, I wonder if a simple CSS fix to VE would suffice for now (or a CSS fix to CodeMirror, assuming it doesn't break anything in non-VE). setupSizing() in ext.CodeMirror.js may be at fault here as well, though I doubt it. That code will be reworked a tiny bit in the near future for T323329.

Change 959246 had a related patch set uploaded (by DLynch; author: DLynch):

[mediawiki/extensions/CodeMirror@master] Update embedded CodeMirror library to 5.65.15

https://gerrit.wikimedia.org/r/959246

Updating CodeMirror does indeed seem to fix it. After being on my backburner for only four months, even.

Minor note for local testing: reproducing this seemed to also depend on the line numbers being hidden, which isn't the default config.

Anyway, this is sort of a naïve updating, in that I built the lib and copied the relevant files over, but have only done basic testing to made sure there's no obvious breakage / console errors. I'd appreciate someone who knows more about the extension giving it a look-over as well...

Change 959246 merged by jenkins-bot:

[mediawiki/extensions/CodeMirror@master] Update embedded CodeMirror library to 5.65.15

https://gerrit.wikimedia.org/r/959246

@DLynch can this be closed ?

@TheDJ not up to me, it's now in the workboard column where @ppelberg reviews tickets before closing them.