Xinerama: different after exiting and entering fvwm [solved]

I am able to use xinerama in fvwm. I use this configuration:

XineramaPrimaryScreen 0 Xinerama on Style * StartsOnScreen 0

I have two screens, an external VGA monitor with a resolution of 1280x1024, and the LCD with 1280x800. The final virtual desktop is 2560x1024. Therefore, there is a region in the 1280x800 LCD monitor lager than the visible area. You could:
b[/b] Move the mouse cursor towards the edges and scroll (“pan”) the display.
b [/b]“If you have display devices with different maximum resolutions you might desire a non-rectangular screen to avoid panning.” (Thomas Winischhofer site). This is achieved by the MergedNonRectangular option in xorg.conf.

I am having a very strange behaviour. When X starts, I get the b[/b] panning behaviour (xdm login screen). In this situation, when I maximise a window, it occupies the whole virtual desktop (both monitors). If I log into fvwm and exit, I am able to get b[/b] MergedNonRectangular option (the one I prefer, because maximised screens stay on one monitor). If I restart X (ctrl-backspace) I am in b[/b] behaviour again. The log file /var/log/Xorg.0.log does not change after exiting and entering back into fvwm.

I do not understand why entering and exiting fvwm could change how xinerama behaves. Any hints appreciated. Thanks!

I have a SiS 771/671 PCIE card. I am using MergedFB and I have followed the indications in Thomas Winischhofer site. This is the device section of my /etc/X11/xorg.conf:

Section "Device" Identifier "Device0" Driver "sis" VendorName "Silicon Integrated Systems [SiS]" BoardName "771/671 PCIE VGA Display Adapter" BusID "PCI:1:0:0" Option "EnableSiSCtrl" "yes" Option "DRI" "off" Option "MergedFB" "auto" Option "MetaModes" "1280x1024-1280x800 1280x1024+800x600 1280x800 800x600" Option "MergedNonRectangular" "on" Option "MergedDPI" "100 100" EndSection

Edit: I solved this issue by commenting the Option “Xinerama” “true” in the Server flags section of xorg.conf. :blush: