XFCE4 screen resolution stuck too low

[Lars Noodén <lars.nooden@gmail.com>]

In XFCE4, on Jessie, I am getting a display resolution of 1400x1050 
instead of 1680x1050.  If I go to the XFCE Menu -> Settings -> Display, 
it gives me only a single choice, that of 1400x1050, not higher or lower.

xrandr seems to be able to see the maximum resolution:

$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 1400 x 1050, current 1400 x 1050, maximum 1400 x 1050
default connected 1400x1050+0+0 0mm x 0mm
   1400x1050     60.00*
  1680x1050 (0x186) 146.250MHz
        h: width  1680 start 1784 end 1960 total 2240 skew    0 clock 
65.29KHz
        v: height 1050 start 1053 end 1059 total 1089           clock 
59.95Hz


What should I be looking at to get this set in XFCE?

Regards,
/Lars