video output with USB Type-C on XPS13 (9350)

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Hi everyone,

I'm having some trouble getting video output on an external monitor over USB Type-C/Thunderbolt, with my Dell XPS13 (9350).

When connected to an external display (via a HDMI or VGA adapter), the external display is listed (with details like manufacturer and screen size) in gnome-control-center display, and the external monitor also appears in xrandr under DP1; however, the external monitor claims there is no video output.

Any ideas? I'd appreciate any and all help or advice.

Here is the output from xrandr:

$ xrandr

Screen 0: minimum 8 x 8, current 3840 x 1200, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 294mm x 165mm
   1920x1080     59.93*+
   1400x1050     59.98  
   1600x900      60.00  
   1280x1024     60.02  
   1280x960      60.00  
   1368x768      60.00  
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   640x480       59.94  
   720x405       60.00  
   640x360       60.00  
DP1 connected 1920x1200+1920+0 (normal left inverted right x axis y axis) 519mm x 324mm
   1920x1200     59.95*+
   1600x1200     60.00  
   1680x1050     59.95  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.08    60.00  
   800x600       75.00    60.32  
   640x480       75.00    60.00  
   720x400       70.08  
   1920x1080     59.93  
   1920x1080_60.00  59.96  
   1920x1200_60.00  59.88  
DP2 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

The last two entries (1920x1080_60.00 and 1920x1200_60.00) were added manually in an attempt to get video output.