New Linux and Mint user here - installed on an old laptop (Acer E5-574T) with 16gb memory and a new 2tb SSD.
Frequently, either after waking the computer or when multiple windows (different applications) are open, the mouse will refuse to change focus when switching between applications. It seems to get 'stuck' on whatever application I was using. I can change applications using keyboard commands, and I can navigate around those applications using the keyboard, but the mouse clicks will only be registered on the original application. Not even clicking on the task bar makes a difference. The only thing that fixes it is a reboot, but it happens again minutes after opening more than one application.
I'm using a Bluetooth logitech mouse, if that has any effect on the solution, but even if I use the touchpad the clicks and inputs are still stuck on the one application.