On my MacBook Pro (Early 2015), I've been struggling with a problem since I upgraded to Sierra.
I don't use Exposé very often, but some times, when I trigger it accidentally with a swipe on the trackpad, it freezes the window manager.
What happens is that the windows start moving inwards (like they do for Exposé) for a beat, but then stops again almost immediately. And after that, the window manager appears to be frozen. I can't bring up any menus, get the “Force Quit” window to appear or change active windows. I can bring up the Cmd-Tab switcher and select a different window, but doing so does nothing – the app that had focus keeps focus, and the windows stay as they were. I can open Siri or Alfred via keyboard shortcuts, but neither receives keyboard focus or reacts to the mouse. I can talk to Siri, but since she can't log me out or kill a process or something like that, that's also useless.
I've only found two ways out of this problem. One is to do a hard reboot (which I'm not fond of), and the other is to SSH in from another machine and do a “sudo killall Dock”, which does alleviate the issue, but requires some logistics.
Any suggestions on how I can stop these freezes, or at least get easier out of them?