'Slab in'
"They USED to invert the left & top with bottom & right to visually indicate clicked or held down. "
I've heard that being called 'Slab in' i.e. trying to visually show that the button has been pressed in. The strange thing is I sometimes find Windows does change the icon to 'Slabed in' but doesn't do the action!
Biting the hand that feeds IT
