I have a 27" iMac Retina (Late 2015, 3.2GHz i5, 24GB, Radeon R9) with HDMI secondary screen on a Thunderbolt/HDMI adapter. I use Mission Control to put 4 workspaces on the main monitor, and switch back and forth using control-arrow keystrokes. This still works, but screen hot corners to view Mission Control, Desktop, etc. do not work any more. Also, I use auto-hide for the task bar, and moving the mouse to the bottom of the screen no longer pulls up the task bar -- as of today, I have to click at the bottom.
Everything had been working fine until a recent High Sierra update (maybe 1-2 updates ago?). When problems first started, I deleted ~/Library/Preferences/com.apple.dock.plist and restarted Dock a few times, and eventually got things working again.
Today, none of that has helped: hot corners doesn't work, auto-hide taskbar only works with a mouse click, and I can't click and move windows after I use a keyboard shortcut to show the Mission Control overview. (I can select a window in the overview and it gets popped to the top of the stack, but I can't click-and-drag windows any more.)
It looks like there's something wrong with the mouse (a Razer connected by USB), but an old generic USB mouse doesn't work any differently. So perhaps the problem is in mouse / UI handling?
I also have a couple of MacBook Pros which have all the same updates (I think) but the magic trackpad shortcuts to show Mission Control haven't had any problems, and I think Bluetooth mice attached to them are working OK (have to check to be 100% sure).
I have had some other trouble with some user input on the iMac: I use VMWare virtual machines, and both Win10 and Linux guests sometimes miss key-up events, so I either get control/alt/shift stuck down, or I get keys repeating (or both). Clicking the affected key a couple of times usually fixes this. I also use VMWare and Parallels on the MBPs with no problems.
Are there any known problems with UI on iMac on recent High Sierra? Any workarounds?