My understanding is that ResourceLoader is only used for bugs in mw.loader, or the ResourceLoader PHP classes.

I have been putting all bugs related to CSSMin (which is the class that does the embedding) and LESS there, see https://bugzilla.wikimedia.org/buglist.cgi?quicksearch=cssmin and https://bugzilla.wikimedia.org/buglist.cgi?quicksearch=less%20css. If I have this wrong, please update them all (and possibly others these simple searches don't find).

Yeah, I was a little hasty in changing it back with that comment. If the scope were just limited to the LESS file itself (e.g. hypothetically, the appearance of the TOC is wrong, fix the LESS file responsible), I think "Skin and page rendering" would be right.

However, this bug kind of intersects with the LESS pipeline, as you said. And it doesn't affect the actual appearance, just performance. So I moved it back.

But I think it can be fixed by just using the embeddable and embed mixins here (whereas bug 54673 proposes to fix it in a general way).

Yeah, I haven't debugged this, but the annotation preserving probably fails because we have two 'background-image' rules and something gets reordered wrong. And yeah, the mixins will probably help, we use them successfully elsewhere.

Change 127139 had a related patch set uploaded by Bartosz Dziewoński:
mediawiki.mixins.less: Correctly embed images in .background-image-svg

https://gerrit.wikimedia.org/r/127139

Change 127139 merged by jenkins-bot:
mediawiki.mixins.less: Correctly embed images in .background-image-svg

https://gerrit.wikimedia.org/r/127139

Change 127580 had a related patch set uploaded by Ori.livneh:
mediawiki.mixins.less: Correctly embed images in .background-image-svg

https://gerrit.wikimedia.org/r/127580

Change 127580 merged by jenkins-bot:
mediawiki.mixins.less: Correctly embed images in .background-image-svg

https://gerrit.wikimedia.org/r/127580

Fixed and backported to 1.23.

My fix caused bug 66091. I submitted a better fix for it now.