Hooray for all those kernel hackers!
Now, the VGA monitor is detected during an existing X session (instead of restarting X), it doesn't flicker anymore, and XFCE's settings show it right away.
1) Plug in the monitor and turn it on. XFCE can already be running.
2) Go to Settings Manager --> Display, click on the monitor, and click the "use this output" checkbox.
[Optional] 3) Go to Settings Manager --> Desktop, and choose a distinctive desktop for the monitor.
[Optional] 4) To use the monitors side-by-side (extended mode) instead of the default mirrored mode, open a terminal window and try:
$ xrandr Screen 0: minimum 320 x 200, current 1366 x 1536, maximum 8192 x 8192 VGA-0 connected 1024x768+0+768 (normal left inverted right x axis y axis) 304mm x 228mm 1024x768 74.9* 75.1 70.1 71.8 60.0 832x624 74.6 640x480 72.8 75.0 66.7 60.0 640x400 70.1 LVDS connected 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 193mm 1366x768 60.0*+ 1152x768 59.8 1024x768 59.9 720x480 59.7 640x480 59.4
From this command, you can see that the displays are named LVDS (laptop) at 1366x768 and VGA-0 (ext monitor) at 1024.768. Let's move VGA-0 to the right of LVDS:
$ xrandr --output VGA-0 --auto --right-of LVDS
The command also works with
--below. Xrandr changes like this are not persistent across restarts, so redo it every time, put it in a bash alias, or script it into the startup.