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obscure

A window is obscured if some other window obscures it. For example, window A obscures window B if:

Both windows are viewable :input-output windows
Window A is higher in the global stacking order than window B
The rectangle defined by the outside edges of window A intersects the rectangle defined by the outside edges of window B

Notice that window borders are included in the calculation, and that a window can be obscured and yet still have visible regions. See occlude (there is a fine distinction between obscure and occlude).

occlude

A window is occluded if some other window occludes it. For example, window A occludes window B if:

Both windows are mapped
Window A is higher in the global stacking order than window B
The rectangle defined by the outside edges of window A intersects the rectangle defined by the outside edges of window B

Notice that window borders are included in the calculation. See obscure (there is a fine distinction between occlude and obscure).

override-shell

A subclass of shell used to override the window manager. This subclass contains pop-up menus and other temporary objects that the user can never resize and so on.