The option to LOCK panels and their objects into place so they cannot be undocked by accident.
Not everyone one of us is young, and some of us have conditions where we're not able to fully control our movements such as advanced arthritis or MS. I often accidently undock an item in the panel, and I mean OFTEN. It is frustrating, and interrupts my workflow.
For accessibility to disabled designers, there really SHOULD be a way to make those docks NOT so easily moved, undocked, etc. Floating panels was fine when I had finger control but now that I'm in early stages of MS--its frustrating that I accidently undock an item from a panel or move the panel by mistake..
I have no known disabilities, but this still happens to me a lot, but only on the left side of the application window. (Actually, may be a vision problem, in part.) But I only have this issue with the tools panel, which I usually use as a single column.
Photoshop 2019 just added this very feature. It would be great if the different product teams worked more closely to keep the interface conventions the same from app to app, so you don't need to remember which does what.