The linked commit actually implemented a way to disable this behavior. The behavior was implemented in c449fe11c99f5fdcc2f100b18f7f95787e5fd442 for T129521.

I think the current default behavior is much saner than if we never hid the floatables. Per the linked task, the main purpose is to make DropdownWidget's floatable MenuSelectWidget disappear when the DropdownWidget is scrolled offscreen (instead of a "detached" MenuSelectWidget remaining visible), which seems like a very reasonable thing to do.

For comparison, if you try to scroll while a <select> dropdown is open, browsers will either immediately close the dropdown or prevent you from scrolling.

See T129521 examples of the results that never hiding dropdowns produces. This specifically:

So there is some argument to use it iff the following two condition are met:

1. The container is a DOM element, and not the viewport - otherwise you are just wasting time doing clipping the browser is already doing.
2. The floatable is *completely* outside the container.

Currently this is causing the Echo notification popup to disappear as soon as I scroll down 1 pixel.

Change 405816 had a related patch set uploaded (by Esanders; owner: Esanders):
[mediawiki/extensions/Echo@master] Don't hide notification popup when scrolling

https://gerrit.wikimedia.org/r/405816

Currently this is causing the Echo notification popup to disappear as soon as I scroll down 1 pixel.

This is fixed by the patch above.

Change 405816 merged by jenkins-bot:
[mediawiki/extensions/Echo@master] Don't hide notification popup when scrolling

https://gerrit.wikimedia.org/r/405816

@matmarex @Esanders Anything left here?