Comment 17 for bug 564463

The solution that Chris mentions above has a high chance of causing regressions for some Intel graphics hardware, and therefore isn't a viable solution. After some Discussion with Chris, there is a possible way to fix it that would require changes in the way the driver does probing, These changes would have to be done with the cooperation of the upstream driver maintainers, and while they are probably worth pursuing, they won't be implemented soon.

I believe that this can be solved by running xrandr so that the driver will use the probes resolution of the external monitor. For that reason, I think that the fix for this is to run xrandr.