Fitts' Law and edge-flipping in window managers

March 25, 2007

Simplified and summarized a lot, Fitts' Law says that the four of the five easiest locations to reach with the mouse are the four corners of the screen, because they require very little precision. (There's a lot more to it than just that, of course; see AskTog.)

A number of window managers, fvwm2 included, have virtual screens and a feature called 'edge flipping': if you move your cursor to a screen edge where there's another virtual screen beyond it and keep 'pushing', the window managers flip you to the next virtual screen. I'm pretty fond of this feature in fvwm2; it's especially handy when moving or resizing windows.

(fvwm2 has a similar feature for Xinerama split screens when placing windows, which is really convenient.)

I have recently come to realize that the problem with edge flipping is that it shoots Fitts' Law in the head. With edge flipping in effect, the corners are usually not easy targets, because shortly after you hit them you flip to a new screen (which generally moves your mouse, among other things).

(Worse, fvwm2's implementation actually steals the edge pixel, so even if you have a very long delay on the edge flipping feature it doesn't help.)

This explains why I like my top left virtual screen so much; I have some useful stuff parked in the top left corner of the screen and in the top left virtual screen, that corner is not stolen by edge flipping. I also suspect that it explains why Gnome and KDE don't have this feature, despite how convenient it looks. (Of course, another reason would be the potential for user confusion: 'suddenly all the windows on my screen disappeared! what happened?' At least with explicit virtual screens the user had to click on something to make all their windows vanish.)

This is kind of depressing, since a few years ago I tried to reorganize my screen layout to make better use of the corners (although I didn't entirely succeed even without edge flipping). I'm not sure what to do; perhaps I should turn off edge flipping entirely for a while and see how much I actually use it and how annoying it feels to live without it.

(Thinking of Fitts' Law also makes me wonder how well the common Gnome and KDE desktops are using the corners these days. I suppose I should experiment and find out.)

Written on 25 March 2007.
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Last modified: Sun Mar 25 22:37:23 2007
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