fvwm uses a virtual desktop. "The screen is a viewport onto a desktop which may be larger than the screen." You can move the screen around over this virtual desktop. Moving down has the same effect as moving all the windows one screen height up. If a window is larger than the screen, you can easily pan around to see other parts of it. This is very handy when your screen height is 600 pixels and $ENTERPRISE_APP draws dialog boxes 1200 pixels tall. It also means I can lay out windows with a total area several times the screen area without having any overlapping.

I wish someone would make a compositing window manager based on a big virtual desktop instead of on shrinking all the windows to illegibility and making me pick one to obscure all the others.