So this started happening for me with 4.25. I was hoping that 4.26 and a clean install of windows would fix it, but sadly it did not.
No idea what’s causing it. My friend runs 4.25 and no issue for them.
Oh whoops. I meant to include this in the original post. It’s every “mode” aside from select. So landscape, paint, foliage, edit. Content browser is totally fine though so I don’t get it
Is it this one menu and / or other slate elements behave like this?
on all modes other than Select
So landscape specific?
The YT upload is in 1080p but are you running a much higher resolution natively? Perhaps you’ve got windows animations enabled in the project preferences - this one makes my editor completely unusable, for example…
Oh, I was really hoping this was the fix but nope. I do run two monitors at once but both at 1080p. It’s never been an issue in previous versions of the editor either.
So, this issue definitely only cropped up as of 4.25. I re-downloaded and installed 4.24 and the issue is not there, so when they made the changes to the different modes, it looks like they somehow broke the UI for apparently only me 
And it’s finally fixed. Hooray!
I simply went to Window - Load Layout and chose the default Editor Layout and this fixed the issue. Apparently my custom layout was causing the issue. Strange, but happy I finally figured it out.