Change 649116 had a related patch set uploaded (by Isarra; owner: Isarra):
[mediawiki/skins/Timeless@master] Redo scrolling tables visuals to be less bad

https://gerrit.wikimedia.org/r/649116

Okay, I have noooo idea if the above will actually fix this or not. Given the stickytableheaders gadget appears totally unaffected (as in, still broken) I'm not hopeful. Then again that might actually be the css not getting along, so maybe cleaned up js will help this?

We'll, uh, see, I guess.

Change 649116 merged by jenkins-bot:
[mediawiki/skins/Timeless@master] Redo scrolling tables visuals to be less bad

https://gerrit.wikimedia.org/r/649116

Alright, confirmed this is still happening, also I have noooo idea how the collapsible is supposed to work to begin with or how the... DOM relates to it. Like... is the collapsing stuff even in a separate distinct element?

rMW resources/src/jquery/ has the source.

Do you have any other, simpler, examples I can look at to try to get to the bottom of this with?

This kind of insanely complicated multilevel lua module makes it a little difficult to figure out what's going on that might even be relevant...

Sorry, that probably came out harsher than I wanted. Just... something simpler I could use to replicate similar output to debug with elsewhere...?

I honestly don't off hand know of a lot of other structures like this one. Not in mainspace today. Infoboxes might go this way sooner or later (for those that support collapsing).

The wikitext equivalent of this would look something like:

<div class="mw-collapsible">
<div class="caption">Caption<div>
<div class="mw-collapsible-content">
{| class="wikitable"
|+ Aggregators
|-
! Aggregator
! Score
|-
| Metacritic
| 90%
|}
</div>
</div>

Add multiple other tables at leisure immediately below the first as desired.