SiS 771/671 and MergedFB Xinerama

[Joanet]

Hi to everybody. I would appreciate if you could give me some hints on
this.

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. 
From the documentation, you could: 
(a) Move the mouse cursor towards the edges and scroll ("pan") the display.
or
(b) "If you have display devices with different maximum resolutions you might 
desire a non-rectangular screen to avoid panning." 
(http://www.winischhofer.eu/linuxsispart2.shtml#mergedfbmode,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 (a) 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 my window 
manager (fvwm) and exit, I am able to get (b) MergedNonRectangular option 
(the one I prefer, because maximised screens stay on one monitor). If I 
restart X (ctrl-backspace) I am in (a) behaviour again. The log 
file /var/log/Xorg.0.log does not change after exiting and entering back into 
my window manager (fvwm).

I do not understand why entering and exiting my window manager would change 
how xinerama behaves. Also, I would like to have the (b) behaviour from the 
beginning, without having to enter and exit my window manager. Any hints 
appreciated. Thanks!

====
Info:
I have a SiS 771/671 PCIE card. I am using MergedFB and I have followed the 
indications in 
http://www.winischhofer.eu/linuxsispart2.shtml#mergedfbmode,Thomas 
Winischhofer site[/url].  This is the device section of 
my /etc/X11/xorg.conf:
[code]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[/code]
-- 
Joanet