Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: TITAN X (Pascal)/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 456.71
Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash: rB0f45cab862b8
Worked: Well, never seen this before! I was using a wacom/cintiq on windows in dual screen setup, with blender set to full screen, and had just used the 'rotate view in 2D' shortcut when I noticed the issue.
Short description of error
Something seems to have become corrupted with this workspace. Panels can't be split, and when it's duplicated, UI doesn't update properly - menus that have been opened don't close (see video), changing workspace doesn't change, dialog boxes don't disappear!
Exact steps for others to reproduce the error
Load the attached file. Notice it doesn't have the option to split the panes in the tab named Layout. Duplicate the tab named Layout. Try the menus and dialog boxes. They open but don't close! Also, switching to the newly duplicated workspace doesn't actually change the workspace, it just keeps whatever workspace was there before! .