I know what's causing this: It's a side effect of us switching back from the "Metal" graphics pipeline to OpenGL on Mac. The reason for doing so is that the UI sometimes freezes 1-2 seconds when opening the full screen review mode when using the Metal pipeline.
If you prefer using the Metal pipeline instead, edit "jAlbum.cfg" (See Tools->Open directories->Program directory), then delete these two lines:
java-options=-Dsun.java2d.metal=false
java-options=-Dsun.java2d.opengl=true