From TRAC 9273 (bert, feb 2009)

4.0.2205, screen scaling enabled: When switching projects, the screen goes black for a while. Same when saving a project, although that takes longer so is more disturbing.

The effect seems to be most visible with a screen size larger than the project size. When switching, the project is first drawn scaled up to the window size, then again at its native size (in the center of the window) while the scaled up version is still visible around it, then the screen is filled with black, then it is filled in dark gray, then the center is filled black, and then the project is drawn in the center again. (I tried in a 5000x3000 VMware window so filling is slow enough that I can see what's happening).