The Adobe Color Themes panel is cut off on the right side. There's no left-right cursor when I hover over either the left or right sides. It expands up and down just fine. Other panels work just fine.
I used Alt+Control+Shift to reset my preferences and that didn't fix it. So right now everything is the default settings.
Which version on which system is it?
Sorry, it's Illustrator 25.1 64-bit on Windows 10.
If you drag the panel out of the dock, does it work better then?
No. It still only lets me resize it vertically.
Just came to post the exact same problem.
Newest version, windows 10 64bit.
Nothing works. Also tried shrinking user interface to lowest settings. Zip.
ADOBE - Your quality control is beyond abismal these days. It seems like you don't test anything.