Unity 2D doesn't load my multi-monitor setup properly at boot

[Michael Bienia]

This is with current unity-2d (5.8.0-0ubuntu1) from current precise.

My monitor setup looks like:
- 17" TFT with 1280 x 1024 on the left
- 24" TFT with 1920 x 1200 on the right (main monitor)

Upon boot (with auto-login) I get since some time a weird setup:
I get one dash panel and one top panel on both monitors (as expected) but the usuable resolution is wrong: instead of a total of 3200 x 1200, it looks like only 1920x1200 (starting from left upper corner) are really used, so my desktop ends around 1/3 of my 24" TFT (the right 2/3 are the purple color from lightdm(?)) and I can't move further right.

Checking the Display panel in gnome-control-center the displayed setup looks like I want it but it doesn't match the reality.
Changing one setting (e.g. turning one monitor to off) and applying it but then clicking "Restore the previous configuration" restores it to my expected configuration.

[Michael Bienia (geser)]

This is how it looks like after booting.

[Michael Bienia (geser)]

This is how it should look like.