Comment 0 for bug 1292467

When booting with two screens (internal LVDS and VGA), lightdm comes up in a mode where it displays separate screen content on both displays (so no mirror mode). These screens seem to be arranged side-by-side regardless of the fact that (like in my case) the combined width can be greater than 2048 and that is not supported with 3D acceleration at least on that older i945GME graphics.

This results in very poor graphics performance and compiz using a lot of cpu cycles (which are rather limited on this Atom N270 anyways). Even worse, this does not get resolved when changing the setup in system settings to either only having one screen active or arranging them on top of each other).

Booting with only the internal screen and then plugging in the external one after login seems to handle this better (although I probably need to remove any previous config to get into a kind of vanilla state again). Also it seems to be ok when I had the dual monitor boot and lightdm coming up side-by-side, when unplugging the external monitor before logging in.

So logging in with the dual-monitor side-by-side setup causes 3D acceleration to be broken for that session (even when it switches to single monitor by configuration). However going back to lightdm, unplugging the external monitor, then log in and reconnect the external monitor is ok.