In a multiple display setup, the cursor shouldn't be allowed to simply go to the other display. The edge should pose some resistance, for example only continually moving the mouse for 10 pixels or more should allow it to pass to the other display. Other method are good as well. Why: the edge is a sweet spot for grabbing a scrollbar, for resizing a window, and maybe others. The only possible problem I see for this is if an application is on two screens, the user would expect the cursor to behave as usual. This would be bad especially for games. I was directed here after submitting this bug to KDE's Kwin (https://bugs.kde.org/show_bug.cgi?id=217116 )
We now support this through XFixes v5 pointer barriers, although it requires client support to avoid the cases you mention: games, or apps spanning multiple monitors. GNOME Shell already takes advantage of this.
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