These are actually "MediaWiki UI" checkboxes, predating OOUI.

This looks like it might be a browser bug, caused by the use of CSS multi-column layout (added in https://gerrit.wikimedia.org/r/c/mediawiki/core/+/747983 a few weeks ago), I recall other issues with it in the past in Chrome.

I've looked into this. The issue seems to be that although we removed the table layout from the columns, the individual checkbox/label pairs are still set to display: table; , which causes them to wrap to the next column.

we can change this by setting checkbox/label pairs to display: flex; which seems to wrap without breaking the inputs.

.mw-ui-checkbox {
     display: flex;
    flex-wrap: wrap;
    margin-bottom: 0.5em;
}

I created a patch to convert the checkboxes to OOUI in order to fix T301500, it looks like that also fixes this.

Change 762077 had a related patch set uploaded (by Bartosz Dziewoński; author: Dylsss):

[mediawiki/core@master] Convert SearchFormWidget namespace checkboxes to OOUI

https://gerrit.wikimedia.org/r/762077

@matmarex That all looks pretty good. One usability must-have issue to fix:

Before	Patched

One nice-to-have: The buttons for selecting/deselecting should be OOUI ones too.

No longer limited to Google Chrome. The problem also extends to Apple Safari, but I'm unsure whether the patch can work also for Safari. My Safari version is 15.3.

The problem seemed interesting to me, so I experimented a little trying to get to the bottom of this.

Here's a small demo that shows what is happening: https://test.wikipedia.org/wiki/T300574

Chrome	Firefox

Chrome supports splitting tables across multiple columns (unlike Firefox). However, the way it picks the "break points" is not very smart; in particular, it will happily break in the middle of a cell, even if this is a single-cell table.

As it happens, each checkbox on Special:Search is actually a tiny table, and that's why they are affected by this. I think I was wrong to call this a bug in T300574#7667793 though, it's just a little weird.

The suggestion to change them into flex boxes from T300574#7701605 probably wouldn't work; as demonstrated above, Chrome is also happy to break columns in the middle of flex boxes.

Changing them into OOUI checkboxes (T300574#7705774) doesn't work either, each one of them is also a tiny table. The column breaking is slightly different and less obviously broken, but it's still not great (T300574#7710681).

However, there is a fix that works – we can just instruct it to not break columns inside of the table cells! I will submit a patch that does this.

Change 765542 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] mediawiki.special.search.styles: Don't break columns inside checkboxes

https://gerrit.wikimedia.org/r/765542

Change 765542 merged by jenkins-bot:

[mediawiki/core@master] mediawiki.special.search.styles: Don't break columns inside checkboxes

https://gerrit.wikimedia.org/r/765542

When will this be deployed? I still see the issue as of date.

It's being deployed this week (note the date in the tag added by the bot: MW-1.38-notes (1.38.0-wmf.24; 2022-02-28)), per the usual schedule: https://wikitech.wikimedia.org/wiki/Deployments/Train#Typical_Deployment_Week

Accordingly, I see the fix at MediaWiki.org, but not yet at English Wikipedia:

https://www.mediawiki.org/w/index.php?title=Special:Search&profile=advanced&search=&fulltext=1
https://en.wikipedia.org/w/index.php?title=Special:Search&profile=advanced&search=&fulltext=1