I have a MacBook Pro 15" Retina. I'm currently on Parallels Desktop for Mac 13.3 and macOS 10.3.4, but this problem has been consistently present since macOS 10.2 and Parallels 11. After I've been working in Windows 10 Guest OS for a few minutes, it will stop responding to mouse clicks, left or right. The cursor still moves, but I can't change focus or activate anything. The trigger is a mystery, but generally happens when I have been working in the host OS and return to the guest. Restarting the guest OS fixes it, but the easier solution I've found is to switch to the host OS with the guest window in the background and move the cursor back and forth over the guest window. This triggers "big cursor", a feature of macOS used to find the cursor. This resets the issue in the guest OS. Reinstalling Parallels Toolbox doesn't help, and the new Toolbox seems to have made the problem worse.