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Currently there are two shortcuts for present windows, present windows from the current desktop and present windows from all desktops. My idea is that, optionally, when you hit the present windows from all desktops shortcut, around the edges of the screen there is a thin partially transparent strip in a user-defined color. The strip would probably be about the thickness of the default panel, although it too could be customizable. If you click this strip or hold your mouse over it for a certain amount of time, it switches from single-desktop to multi-desktop mode. The window previews would shrink and the window previews would fly in from the other desktops. If you click or mouse over the bar again, it switches back to single-desktop mode.

Assign a color to each desktop (e.g average for wallpaper), then use these color boxes as buttons to select the present desktop. A flag like (button with all colors) would show windows from all desktops.

Users could also create sets of 1+2 second and 3+4 desktops, if they like

I'm concerned that would be too complicated. The point is to have a large and easy click target so people don't have to think about it too much.