The broken layout is caused by loading the styles for legacy Vector while viewing new Vector, and easily reproducible by using mw.loader.load('skins.vector.styles.legacy') in browser console – however I can't figure out what would cause that to happen in DiscussionTools, and I can't reproduce this.

I thought that we might be calling the action=parse API with useskin=vector, which would mean the legacy Vector and maybe load the wrong styles, but that should not be happening since https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/759531 (a few weeks ago).

Possibly this is an issue like T288113 or T300278, except "backwards".

@matmarex: are you able to estimate how many people/Reply Tool "uses" this issue might be impacting?

...I ask the above thinking the potential causes you named in T303265#7760060 might provide us enough information to help us estimate this.

No. All I know is that two people reported it on that talk page, and I wasn't able to reproduce.

In T303265#7762186, @matmarex wrote:

No. All I know is that two people reported it on that talk page, and I wasn't able to reproduce.

Understood.

As we talked about during yesterday's team meeting, we're going to wait on prioritizing work on this task until more instances of this issue are reported so that we can take another pass at identifying a common cause/pattern among them.

Maybe anyone can adjust any browser's window size, e.g. Chrome's, to better reproduce the issue?

Just wanted to note that this happened before the script for T299104 was run. At this point in time, there was a known bug where we served the wrong Vector to certain users which has since been fixed.