maximized Firefox window draws shadow on the 2nd screen
Say you have 2 screens arranged vertically. When you maximize any window on the screen below, nothing appears on the screen on the top. Normally, logically. Except when that window is Firefox. When Firefox is maximized on the screen below, a shadow displays at the bottom of the screen on top, see the attached.
Other chromium-based browsers don't display this shadow, other windows of other software do not, the shadow shown by Firefox only displays when the window is maximized, not regular. And displays no matter the theme (here in the picture is using the default, which shows a titlebar like Windows' default one).
Any idea how to fix it?