Attempting to assign a material to a layer can create two, permanent, black squares on one (or more) otherwise invisible topmost windows. Note: the topmost window will capture mouse clicks and will bring Rhino to the top of the zorder. It is bigger than the visible rectangles.
have two or more screens with desktop extended across them - my primary monitor is 1080p and sits to the right of the secondary - the secondary is 4k
make sure your main monitor’s background is sufficiently not-black so so you can see black squares when they appear
open rhino (you don’t need to create a file)
move rhino to the secondary monitor and maximize it (not sure if that is necessary, just do it)
select layer panel and click on the circle that displays the dialog that assigns a material to a layer
the dialog is displayed and things are still fine
drop down the material selector, uh, dropdown
you will see the first black box appear somewhere over the top right quadrant of your main monitor
select create new material, pick a type of material
the second black box appears
Finished how to recreate
These rectangles last the life of your rhino app. If you have more than one rhino running then I think you have to close them all to get rid of the rectangles.
Please see the, somewhat meta, screen cap below.
The evil influence of the black rectangles extends beyond their visible regions but doesn’t occupy the entire top quadrant of the screen. For example: I can successfully click the search icon but clicking over the number 27 will cause rhino to come to the top of the z order and the click will not reach the browser.