If I set a label to have a cursor using GuiCtrlSetCursor then the first time a subclassed context menu appears its background is black. If you right click a second time to get the context menu redrawn then the background is changed to normal.
Here is an example which is a copy of a post by Siao with 2 lines added. If you comment out the GuiSetCursor for the label then the problem goes. This problem was in a completely different script, and the context menu wasn't for an input, but this is about