Unfortunately the display resolution of my laptop never matches that of the other display, and Linux tends to choose 1024x768 as the highest compatible resolution. This is of course totally useless for doing any real work.
My preferred solution for this problem is to use X scaling to bridge the resolution gap between the different screens.
Since none of the regular display configuration tools support scaling, I ended up typing this line very often:
xrandr --output LVDS1 --mode 1600x900 --output HDMI3 --mode 1920x1080 --scale-from 1600x900
Eventually I got fed up and decided to automate the process, the result is automirror, a little Bash script that automatically configures all attached displays in a mirror configuration. automirror is available on https://github.com/schlomo/automirror.
Typical Use CasesConnecting a Full HD 1920x1080 display via HDMI to my 1600x900 laptop. In this case automirror will simply configure the HDMI device with 1920x1080 and scale the 1600x900 laptop display. As a result I stay with the full resolution on my laptop display and it also looks nice on the projector.
Another case is where I work with a 1920x1200 computer monitor and add the 1920x1080 projector as a second display. Again the common resolution offered by both devices is 1024x768. automirror will recognize my 1920x1200 display as primary display and scale it to 1920x1080 on the secondary display, which is not really noticeable.
It is recommended to configure a hot key to run automirror so that one can run it even if the display configuration is heavily mwessed up. In rare cases it might be neccessary to run automirror more than once so that xrandr will configure the displays correctly.