
TrenchBroom V2020.1 on Windows 10
Resizing the panels in the entity tab creates a floating bar. See Gif.
Resize that panel by dragging the bar up and down.
Does it happen with 2020.2?
Yes, same with 2020.2 here.
What is your video card? Using Intel, AMD, or Nvidia?

Nvidia Geforce GTX 1060
Thanks, I can reproduce it. Configure the top 2 splitters to be as small as possible, like this:
Then grab the "Entity Browser" splitter and raise it; it will let you shrink the table smaller than it should