[...]
>>>>
>>>> Which series video card are you using? (nVidia, ATI, Intel...) Which driver?
>>>>
[...]
>>>
>>> 00:02.0 VGA compatible controller: Intel Corporation 82915G/GV/910GL
>>> Integrated Graphics Controller (prog-if 00 [VGA controller])
[...]
>>> Kernel modules: i915
>>
>> Ok, so you're using an Intel graphics chipset, managed via the
>> kernel's i915 module. According to Wikipedia, the 82915G, GV and GL
>> models are circa 2004/2005, and so are very old. It's very possible
>> you've encountered a regression. I had something similar happen a few
>> years ago with my 845-based system.
>>
>> Unfortunately, I don't know where you should take this from here.
>> x11-drivers/xf86-video-intel
>
> I was just poking around my /usr/portage/x11-drivers directory and I saw
> that my x11-drivers/xf86-video-intel version 2.17.0-r3 was installed
> yesterday and it thinks I should downgrade. I'm going to try
> downgrading. I'll let you know if it works...
>
Or try the next higher: 2.18.0
Maybe the regression was already fixed.