This seems like fairly bad breakage, so I'm proposing it as current work.

I can't reproduce using Firefox 69.0b8 on Windows 10.

Quick screen recording where I copy-paste a heading and interact with it, it seems to work fine:

@matmarex thank you for checking. Maybe it's macOS specific?

Here's a screen recording showing what I see.

Thanks for the recording. The page looks weirdly narrow – do you have some browser extension or user script that does this? Maybe that is interfering with VE somehow?

If you copy-paste a heading from some other page than Wikipedia into VE, does it also exhibit these issues? For example, try copying this one:

heading

Thanks!

In T232502#5480875, @matmarex wrote:

Thanks for the recording. The page looks weirdly narrow – do you have some browser extension or user script that does this? Maybe that is interfering with VE somehow?

Ahh, right, I forgot I have user CSS. But it doesn't seem to be the cause here; I tested on the Beta cluster, without it, and see the same issue

If you copy-paste a heading from some other page than Wikipedia into VE, does it also exhibit these issues?

Yes—I actually originally discovered this pasting text from another website.