I often experience this issue: when I am running Mathematica and open a different application (such as Skype) or I try to save a plot from my notebook, Mathematica generates a new black screen (see screenshot). If I try to drag the black screen to the main desktop, it simply disappears. Most of the times, the notebook doesn't actually crash and I can still keep using it.
I only have this issue with Mathematica and no other application. This happens mostly (but not only) when I am in full screen. I am now running Mathematica 11.2 and macOS 10.13.2, although I have been experiencing this with previous versions of both of them. Any ideas?