Comment 92 for bug 295192

(In reply to comment #45)
> Note that only on multiple-monitor systems are we apparently prepared to take
> the perf hit of not caching the screen dimensions (in that case simply dragging
> the window from one screen to the other will update the window.screen values).

On Linux (Ubuntu 8.04, Firefox 3), using xrandr to add another screen does not update window.screen properties. So the system can start out being a single-monitor one but change to be multi-monitor, and in this case we should throw out the cache.

See also bug 29100 which is triggered by this, and the related upstream Ubuntu bug at https://bugs.launchpad.net/ubuntu/+source/firefox-3.0/+bug/229298