Mouse left-click didn't work in Setup at all, so I had to run Try mode to install 20.04. Oddly the mouse click worked in Try mode but not in normal setup.
Once 20.04 was installed, left-click became intermittent. If I alt-tab between windows, clicking will work temporarily but then if I try to click on some other window it won't work. I've read things about invisible windows interfering? This happened on a completely clean install (even in setup!) before I installed anything. There are a TON of posts about this same problem in various forums and I have tried every solution, none of them work.
The only thing that makes the mouse work temporarily is "sudo service gdm restart" but A) that only works until the next reboot, and B) that makes the forward & back mouse buttons stop working which is maddening.
This happens with several brands/models of mouse, on various USB ports, using 2 different USB controllers (chipset and ASMedia).
Asus Rampage VI Extreme X299 with GeForce GTX 1660 and Samsung NVMe. No other devices/cards. This is a desktop so there is no touchpad and I do not have multiple pointing devices. I have also tried a different keyboard.
[EDIT] I ran setup on a completely different system, an MSI Intel Z370, and the mouse left-click is broken. Is Ubuntu 20.04 mouse support completely borked?