Preface: I have a laptop with Ubuntu 16.04 / Windows 10 in dualboot and 2 external displays, one with VGA input, and another with HDMI.
Various times I'm using HDMI display as the second on Windows or Ubuntu, and everything is fine.
When I connect VGA display using Windows, all is well too. But the problem is connecting VGA display with Ubuntu,so, I can use just internal laptop screen or external VGA screen, or even mirror video on both screens, but when I try to use extend mode, all that I see is a black screen on one or another display :(
P.S. My graphics card is AMD Radeon HD 7600m, I'm using open source Radeon driver. Another weird thing: internal display resolution is 1366x768 and external VGA is 1280x1024. When I set 1024x768 instead of usual 1280x1024, it works properly. But using any other resolution leads to black screen.
Screen 0: minimum 320 x 200, current 2390 x 768, maximum 16384 x 16384 eDP connected primary 1366x768+1024+0 (normal left inverted right x axis y axis) 344mm x 194mm 1366x768 59.93*+ 1280x720 59.86 1152x768 59.78 1024x768 59.92 800x600 59.86 848x480 59.66 720x480 59.71 640x480 59.38 VGA-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 338mm x 270mm 1280x1024 60.02 + 75.02 1280x960 60.00 1152x864 75.00 1024x768 75.08* 70.07 60.00 832x624 74.55 800x600 72.19 75.00 60.32 56.25 640x480 75.00 72.81 66.67 60.00 720x400 70.08 HDMI-0 disconnected (normal left inverted right x axis y axis)