Hmm, interesting. I don't think that we can do this for all checkboxes everywhere (I think the possible problem in the case where a label would really need to wrap is worse than the problem of unnecessary wrapping we have now, and I personally won't volunteer to audit all of our codebase for this ;) ), but we can probably do it for the watchlist ones.

Change 254267 had a related patch set uploaded (by Bartosz Dziewoński):
SpecialWatchlist: Keep checkboxes and their labels together

https://gerrit.wikimedia.org/r/254267

Change 254267 merged by jenkins-bot:
SpecialWatchlist: Keep checkboxes and their labels together

https://gerrit.wikimedia.org/r/254267

@matmarex: Above I had limited the issue to all forms (with floating lines of checkboxes or other form elements).

In many cases, there is not a floating line of inline items like single words and checkboxes between, but a kind of vertical list with exactly one checkbox in the beginning of a line, line break, and next checkbox at beginning of a new line. Those are not affected.

Currently I don't recall other active forms with such floating line layout, but you might want to perform the same exercise on the sibling: Recentchanges. Some log pages, contributions or allpages might be affected in future when equipped with other elements. Any list with filtering options might be converted from links (which are not kept together with something else) into floating line of form elements and adjacent label.

RecentChanges still uses the list of links (see T119084), but when/if it gets changed, someone should copy the solution from Watchlist.

I don't know any other forms that have this problem, either. I know a few that already use your solution (ProtectedPages, Controbutions).