Original report at:  https://bugs.launchpad.net/bugs/437173
Look like I've forgot for years to report upstream. Sorry :(

Used default of the intel driver: 

when booting a laptop with an external monitor connected, the
default is mirror mode with a resolution that is 'biggest' one
supported by _both_ monitors.  (Compare: http://keithp.com/blogs/X_output_status_july_2008/)

Problem:

The visual impression is often (always in my experience) very bad:

 * external monitor often do not support the naive resolution of the laptop
   display
 * Even worse: first common resolution supported by both monitors has not
   the same aspect ratio as the displays

Suggested solution for same aspect ratio:

...much better would be: If both monitors have the same aspect ratio (e.g. 16:10 or 4:3) choose the smaller resolution as the common base and scale to the size bigger monitor. (Because only one monitor needs scaling it should even save
tons of GPU cycles.

With the proposed algorithm, one output runs with the 'perfect' native resolution and only one is transformed, with taspect ratio is presered! Currently base resolution is: lower than min. of both display, _both_ are transformed, aspect ratio is not kept.


At https://bugs.launchpad.net/bugs/437173 you can find lots of xorg.log
and xrandr output.   Lemme know if you need more other input.

Thx,
Achim