dconf write /org/gnome/settings-daemon/plugins/xrandr/active true
did nothing and arandr looks pretty incomprehensible to me.
If you are otherwise stymied, it is worth giving xrandr a try. The documentation is here: http://www.thinkwiki.org/wiki/Xorg_RandR_1.2#First_discover_what_we_have
And I was able to fix this by doing:
xrandr --current
xrandr --output VGA1 --off
xrandr --output VGA1 --auto --right-of LVDS1
Let me explain these:
The first command is to see what displays I have connected.
$ xrandr --current
Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192
LVDS1 connected 1280x800+0+0 (normal left inverted right x axis y axis) 261mm x 163mm
1280x800 60.0*+
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA1 connected 1920x1080+1280+0 (normal left inverted right x axis y axis) 509mm x 286mm
1920x1080 60.0*+
1280x1024 60.0
1440x900 59.9
1280x800 59.8
1152x864 75.0
1024x768 70.1 60.0
800x600 60.3 56.2
640x480 66.7 60.0
720x400 70.1
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
There is much extraneous data, but I only need the names of the two displays. I see:
LVDS1 connected
and
VGA1 connected
And I think it is safe to assume that VGA is the external monitor. So then I turn it off
xrandr --output VGA1 --off
and back on again
xrandr --output VGA1 --auto --right-of LVDS1
and when I move my cursor to the right edge of the laptop screen, it appears on the external monitor. Success.