I've seen this in a number of situations under beta 4. The buttons in a dialog sheet cannot be cicked on--sometimes a keyboard equivalent works (typically Return and Escape)--but not always.
Example: System Preferences > Security & Privacy > General > Require password [n time] after sleep or screen saver begins. Changing this brings up a confirmation dialog sheet the buttons of which do not respond to a mouse click. System Preferences must be force-quit.
I'm surprised there aren't more reports on this. I have the same problem, regardless of using bluetooth keyboard and trackpad, or screen sharing.
I suspect it has something to do with blocking synthetic clicks, but right now it is blocking all clicks. On some sheets not even the keyboard trick works, for example, trying to turn on FileVault.
I read a hint somewhere and turned off the "Reduce Motion" option under Accessibility -> Display. This allows some of the buttons in Preferences to be clicked, but not all, e.g. turning on FileVault still does not work.
