Undo while space panning

[Formerly Kevin Yin, now disabled]

[20:56] <LiamW> kyin: I noticed some more stuff while reading the code
[20:56] <LiamW> The canvas tries to block undo while panning
[20:56] <LiamW> But it doesn't block undo when space-panning
[20:57] <LiamW> If you could try to get that to crash that would be great
[20:59] <kyin> no crashing seen
[21:09] <kyin> LiamW: I see that undoing while panning can result in long, long redraw times
[21:10] <LiamW> kyin: I saw that too
[21:10] <LiamW> It keeps aborting the forced redraw crap
[21:13] <kyin> panning while redrawing: http://i.imgur.com/iJrz088.png
[21:14] <LiamW> I think I know exactly what causes that

some lines removed from the chat for brevity.