I've built gimp 2.2 on my system and since added a second head. I find that if I create a large image and display it on my secondary head (1600x1200) I cannot draw past 1024 pixels horizontally. If I try, the cursor snaps back to the left, leaving a horizontal line if I use a painting tool.
on the primary head (1024x768, to the left of the 1600x1200 head) I can paint as normal, since the screen is only 1024 pixels wide.
This is clearly a bug, even in the non-xinerama case since the highest pixel I should be allowed to reach in that case would be 1023, without scrolling, surely?
Hope this helps. I dont have time to fix this one myself at present, Im sure its a simple fix.
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