This seems to specifically be related to the Ace editor component used in a MWAceEditorWidget, and seems to be tied to how the Ace editor is deciding how wide characters are. The line height seems to be off as well:

It's a lot more obvious in Vector 2022, but still happens in old Vector (just taking most of a line for noticeable drift to occur).

Anything which triggers a re-render of the editor component after it's opened seems to fix it for the lifetime of that editor instance -- e.g. click into the "options" tab and then back to "formula".

Could be caused by the update in T324609: Upgrade shipped version of the Ace library from 1.4.12 to a newer release (1.13.x?).

It seems that the issue occurs because the editor is set up while the dialog opening animation is still playing. During setup Ace computes font metrics, which it uses later to display the cursor etc., and the update changed how it does this (see this.$measureSizes = function in https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/CodeEditor/+/941cc7c109d34947e727df49ab059bc07ea31426%5E%21/), and while the previous method seems to have worked fine despite the animation, the new one thinks the font size is about half of what it really is.

There's an issue open for it on Ace's repo, it seems: https://github.com/ajaxorg/ace/issues/5012

Ace has committed a fix for it, so we could presumably either backport that into our bundled version or await a new release.

It looks like they release around once a month, and the last release was 3 weeks ago, so I say let's wait.

v1.15.2 is available now and should fix this bug.

I think we are good.

Yes, it seems fixed now. I tested on Windows/Firefox and Windows/Chrome.