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I'm running Ubuntu 17.10 on an iMac retina (update: upgraded now to 18.04, nothing changed), with a Dell as second monitor. The default resolution of the iMac (first monitor) is 3840x2160 (16:9). At this point everything is so tiny it's unreadable, but at least I see the second monitor just fine (resolution 1920x1200 (16:10)).

When I change the resolution of the iMac to 1920x1080 (16:9) the second monitor goes blank. The windows that were positioned on it are still there (and not visible), and I can move the mouse there (not visible).

I use All Settings | Displays to change the settings. After a change, it asks to keep it or not, and whatever I answer, the second monitor never comes back. It remains unusable after reboot too.

I don't know if it's relevant, but I installed Ubuntu on an external SSD disk running on a non-retina iMac, and then later I replaced the iMac with retina. Everything seems to be working fine (so far) except the display resolution.

How can I fix or debug this?

Some lines from /var/log/syslog that might be relevant:

OpenGL renderer string: AMD Radeon (TM) RX Graphics (AMD POLARIS11 / DRM 3.18.0 / 4.13.0-45-generic, LLVM 5.0.0)
OpenGL version string:  3.0 Mesa 17.2.8
Not software rendered:    yes
Not blacklisted:          yes
GLX fbconfig:             yes
GLX texture from pixmap:  yes
GL npot or rect textures: yes
GL vertex program:        yes
GL fragment program:      yes
GL vertex buffer object:  yes
GL framebuffer object:    yes
GL version is 1.4+:       yes
Unity 3D supported:       yes

Lines matching RADEON:

$ cat /var/log/syslog | grep RADEON | sed -e 's/.*\]: //'
(II) RADEON: Driver for ATI/AMD Radeon chipsets:
#011ATI FirePro V7760 (FireGL), ATI Mobility RADEON HD 4850,
#011ATI Mobility RADEON HD 4850 X2, ATI FirePro RV770,
#011ATI Mobility RADEON HD 4870, ATI Mobility RADEON M98,
#011ATI FirePro M5750, ATI RV730XT [Radeon HD 4670], ATI RADEON E4600,
#011ATI RADEON E2400, ATI FireMV 2260, ATI RV670, ATI Radeon HD3870,

Lines matching AMDGPU:

$ cat /var/log/syslog | grep AMDGPU | sed -e 's/.*\]: //'
(II) AMDGPU: Driver for AMD Radeon:
(II) AMDGPU(0): Creating default Display subsection in Screen section
(==) AMDGPU(0): Depth 24, (--) framebuffer bpp 32
(II) AMDGPU(0): Pixel depth = 24 bits stored in 4 bytes (32 bpp pixmaps)
(==) AMDGPU(0): Default visual is TrueColor
(==) AMDGPU(0): RGB weight 888
(II) AMDGPU(0): Using 8 bits per RGB (8 bit DAC)
(--) AMDGPU(0): Chipset: "AMD Radeon (TM) RX Graphics" (ChipID = 0x67ef)
(II) AMDGPU(0): glamor detected, initialising EGL layer.
(==) AMDGPU(0): TearFree property default: auto
(II) AMDGPU(0): KMS Pageflipping: enabled
(II) AMDGPU(0): Output eDP has no monitor section
(II) AMDGPU(0): Output DisplayPort-0 has no monitor section
(II) AMDGPU(0): Output DisplayPort-1 has no monitor section
(II) AMDGPU(0): Output DisplayPort-2 has no monitor section
(II) AMDGPU(0): EDID for output eDP
(II) AMDGPU(0): Manufacturer: APP  Model: ae19  Serial#: 425818683
(II) AMDGPU(0): Year: 2016  Week: 33
(II) AMDGPU(0): EDID Version: 1.4
(II) AMDGPU(0): Digital Display Input
(II) AMDGPU(0): 10 bits per channel
(II) AMDGPU(0): Digital interface is DisplayPort
horiz.: 48  vert.: 27
(II) AMDGPU(0): Gamma: 2.20
(II) AMDGPU(0): DPMS capabilities: Off
(II) AMDGPU(0): Supported color encodings: RGB 4:4:4
(II) AMDGPU(0): First detailed timing is preferred mode
(II) AMDGPU(0): Preferred mode is native pixel format and refresh rate
(II) AMDGPU(0): redX: 0.680 redY: 0.320   greenX: 0.254 greenY: 0.699
(II) AMDGPU(0): blueX: 0.149 blueY: 0.056   whiteX: 0.310 whiteY: 0.329
(II) AMDGPU(0): Manufacturer's mask: 0
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 533.2 MHz   Image Size:  475 x 267 mm
(II) AMDGPU(0): h_active: 3840  h_sync: 3888  h_sync_end 3920 h_blank_end 4000 h_border: 0
(II) AMDGPU(0): v_active: 2160  v_sync: 2163  v_sync_end 2168 v_blanking: 2222 v_border: 0
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 138.5 MHz   Image Size:  475 x 267 mm
(II) AMDGPU(0): h_active: 1920  h_sync: 1968  h_sync_end 2000 h_blank_end 2080 h_border: 0
(II) AMDGPU(0): v_active: 1080  v_sync: 1083  v_sync_end 1088 v_blanking: 1111 v_border: 0
(II) AMDGPU(0): Monitor name: iMac
(II) AMDGPU(0): Serial No: 6C466633B7A21
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 655.3 MHz   Image Size:  3381 x 2816 mm
(II) AMDGPU(0): h_active: 602  h_sync: 656  h_sync_end 1280 h_blank_end 762 h_border: 11
(II) AMDGPU(0): v_active: 1696  v_sync: 1699  v_sync_end 1699 v_blanking: 2416 v_border: 17
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 66.6 MHz   Image Size:  3381 x 2816 mm
(II) AMDGPU(0): h_active: 86  h_sync: 127  h_sync_end 719 h_blank_end 180 h_border: 11
(II) AMDGPU(0): v_active: 2720  v_sync: 2723  v_sync_end 2723 v_blanking: 2880 v_border: 17
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 66.6 MHz   Image Size:  0 x 0 mm
(II) AMDGPU(0): h_active: 0  h_sync: 0  h_sync_end 0 h_blank_end 0 h_border: 0
(II) AMDGPU(0): v_active: 0  v_sync: 0  v_sync_end 0 v_blanking: 0 v_border: 0
(II) AMDGPU(0): Number of EDID sections to follow: 1
(II) AMDGPU(0): EDID (in hex):
(II) AMDGPU(0): #01100ffffffffffff00061019ae3b7a6119
(II) AMDGPU(0): #011211a0104b5301b78200055ae5241b326
(II) AMDGPU(0): #0110e4f5400000001010101010101010101
(II) AMDGPU(0): #0110101010101014dd000a0f0703e803020
(II) AMDGPU(0): #0113500db0b1100001a1a3680a070381f40
(II) AMDGPU(0): #01130203500db0b1100001a000000fc0069
(II) AMDGPU(0): #0114d61630a2020202020202020000000ff
(II) AMDGPU(0): #01100364334363636333342374132310122
(II) AMDGPU(0): #011020325c0230907078301000070fa1000
(II) AMDGPU(0): #01100127631fc78fbb202108864d469fa10
(II) AMDGPU(0): #01100f6f6f7f8feff5aa020a0d062367030
(II) AMDGPU(0): #011203500db0b1100001a565e00a0a0a029
(II) AMDGPU(0): #0115030203500db0b1100001a0000000000
(II) AMDGPU(0): #01100000000000000000000000000000000
(II) AMDGPU(0): #01100000000000000000000000000000000
(II) AMDGPU(0): #0110000000000000000000000000000001c
(II) AMDGPU(0): Printing probed modes for output eDP
(II) AMDGPU(0): Modeline "3840x2160"x60.0  533.25  3840 3888 3920 4000  2160 2163 2168 2222 +hsync -vsync (133.3 kHz eP)
(II) AMDGPU(0): Modeline "1920x1200"x60.0  193.48  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync (74.6 kHz)
(II) AMDGPU(0): Modeline "1920x1080"x60.0  173.11  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync (67.2 kHz)
(II) AMDGPU(0): Modeline "1920x1080"x59.9  138.50  1920 1968 2000 2080  1080 1083 1088 1111 +hsync -vsync (66.6 kHz e)
(II) AMDGPU(0): Modeline "1600x1200"x60.0  161.23  1600 1712 1880 2160  1200 1203 1207 1245 -hsync +vsync (74.6 kHz)
(II) AMDGPU(0): Modeline "1680x1050"x60.0  146.36  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync (65.3 kHz)
(II) AMDGPU(0): Modeline "1400x1050"x60.0  121.79  1400 1488 1632 1864  1050 1053 1057 1089 -hsync +vsync (65.3 kHz)
(II) AMDGPU(0): Modeline "1280x1024"x59.9  109.10  1280 1368 1496 1712  1024 1027 1034 1063 -hsync +vsync (63.7 kHz)
(II) AMDGPU(0): Modeline "1440x900"x60.0  106.68  1440 1528 1672 1904  900 903 909 934 -hsync +vsync (56.0 kHz)
(II) AMDGPU(0): Modeline "1280x960"x60.0  101.34  1280 1360 1488 1696  960 963 967 996 -hsync +vsync (59.8 kHz)
(II) AMDGPU(0): Modeline "1280x854"x60.0   89.34  1280 1352 1480 1680  854 857 867 887 -hsync +vsync (53.2 kHz)
(II) AMDGPU(0): Modeline "1280x800"x60.0   83.71  1280 1352 1480 1680  800 803 809 831 -hsync +vsync (49.8 kHz)
(II) AMDGPU(0): Modeline "1280x720"x60.0   74.65  1280 1344 1472 1664  720 723 728 748 -hsync +vsync (44.9 kHz)
(II) AMDGPU(0): Modeline "1152x768"x59.9   71.95  1152 1216 1328 1504  768 771 781 798 -hsync +vsync (47.8 kHz)
(II) AMDGPU(0): Modeline "1024x768"x59.9   63.53  1024 1072 1176 1328  768 771 775 798 -hsync +vsync (47.8 kHz)
(II) AMDGPU(0): Modeline "800x600"x60.0   38.31  800 832 912 1024  600 603 607 624 -hsync +vsync (37.4 kHz)
(II) AMDGPU(0): Modeline "848x480"x59.9   31.65  848 872 952 1056  480 483 493 500 -hsync +vsync (30.0 kHz)
(II) AMDGPU(0): Modeline "720x480"x59.9   26.85  720 744 808 896  480 483 493 500 -hsync +vsync (30.0 kHz)
(II) AMDGPU(0): Modeline "640x480"x59.9   23.98  640 664 720 800  480 483 487 500 -hsync +vsync (30.0 kHz)
(II) AMDGPU(0): EDID for output DisplayPort-0
(II) AMDGPU(0): EDID for output DisplayPort-1
(II) AMDGPU(0): Manufacturer: DEL  Model: a0bc  Serial#: 827732051
(II) AMDGPU(0): Year: 2017  Week: 18
(II) AMDGPU(0): EDID Version: 1.3
(II) AMDGPU(0): Digital Display Input
horiz.: 52  vert.: 32
(II) AMDGPU(0): Gamma: 2.20
(II) AMDGPU(0): DPMS capabilities: StandBy Suspend Off
(II) AMDGPU(0): Supported color encodings: RGB 4:4:4 YCrCb 4:4:4
(II) AMDGPU(0): First detailed timing is preferred mode
(II) AMDGPU(0): redX: 0.660 redY: 0.332   greenX: 0.302 greenY: 0.613
(II) AMDGPU(0): blueX: 0.150 blueY: 0.063   whiteX: 0.313 whiteY: 0.329
(II) AMDGPU(0): Supported established timings:
(II) AMDGPU(0): 720x400@70Hz
(II) AMDGPU(0): 640x480@60Hz
(II) AMDGPU(0): 640x480@75Hz
(II) AMDGPU(0): 800x600@60Hz
(II) AMDGPU(0): 800x600@75Hz
(II) AMDGPU(0): 1024x768@60Hz
(II) AMDGPU(0): 1024x768@75Hz
(II) AMDGPU(0): 1280x1024@75Hz
(II) AMDGPU(0): Manufacturer's mask: 0
(II) AMDGPU(0): Supported standard timings:
(II) AMDGPU(0): #0: hsize: 1152  vsize 864  refresh: 75  vid: 20337
(II) AMDGPU(0): #1: hsize: 1280  vsize 1024  refresh: 60  vid: 32897
(II) AMDGPU(0): #2: hsize: 1600  vsize 1200  refresh: 60  vid: 16553
(II) AMDGPU(0): #3: hsize: 1920  vsize 1080  refresh: 60  vid: 49361
(II) AMDGPU(0): #4: hsize: 1920  vsize 1200  refresh: 60  vid: 209
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 154.0 MHz   Image Size:  518 x 324 mm
(II) AMDGPU(0): h_active: 1920  h_sync: 1968  h_sync_end 2000 h_blank_end 2080 h_border: 0
(II) AMDGPU(0): v_active: 1200  v_sync: 1203  v_sync_end 1209 v_blanking: 1235 v_border: 0
(II) AMDGPU(0): Serial No: 7MT017531V0S
(II) AMDGPU(0): Monitor name: DELL U2415
(II) AMDGPU(0): Ranges: V min: 49 V max: 61 Hz, H min: 30 H max: 83 kHz, PixClock max 175 MHz
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 148.5 MHz   Image Size:  518 x 324 mm
(II) AMDGPU(0): h_active: 1920  h_sync: 2008  h_sync_end 2052 h_blank_end 2200 h_border: 0
(II) AMDGPU(0): v_active: 1080  v_sync: 1084  v_sync_end 1089 v_blanking: 1125 v_border: 0
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 74.2 MHz   Image Size:  518 x 324 mm
(II) AMDGPU(0): h_active: 1920  h_sync: 2008  h_sync_end 2052 h_blank_end 2200 h_border: 0
(II) AMDGPU(0): v_active: 540  v_sync: 542  v_sync_end 547 v_blanking: 562 v_border: 0
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 74.2 MHz   Image Size:  518 x 324 mm
(II) AMDGPU(0): h_active: 1280  h_sync: 1390  h_sync_end 1430 h_blank_end 1650 h_border: 0
(II) AMDGPU(0): v_active: 720  v_sync: 725  v_sync_end 730 v_blanking: 750 v_border: 0
(II) AMDGPU(0): Supported detailed timing:
(II) AMDGPU(0): clock: 27.0 MHz   Image Size:  518 x 324 mm
(II) AMDGPU(0): h_active: 720  h_sync: 736  h_sync_end 798 h_blank_end 858 h_border: 0
(II) AMDGPU(0): v_active: 480  v_sync: 489  v_sync_end 495 v_blanking: 525 v_border: 0
(II) AMDGPU(0): Number of EDID sections to follow: 1
(II) AMDGPU(0): EDID (in hex):
(II) AMDGPU(0): #01100ffffffffffff0010acbca053305631
(II) AMDGPU(0): #011121b010380342078ea0495a9554d9d26
(II) AMDGPU(0): #011105054a54b00714f8180a940d1c0d100
(II) AMDGPU(0): #011010101010101283c80a070b023403020
(II) AMDGPU(0): #011360006442100001e000000ff00374d54
(II) AMDGPU(0): #0113031373533315630530a000000fc0044
(II) AMDGPU(0): #011454c4c2055323431350a2020000000fd
(II) AMDGPU(0): #01100313d1e5311000a20202020202001bf
(II) AMDGPU(0): #011020322f14f9005040302071601141f12
(II) AMDGPU(0): #011132021222309070765030c0020008301
(II) AMDGPU(0): #0110000023a801871382d40582c45000644
(II) AMDGPU(0): #0112100001e011d8018711c1620582c2500
(II) AMDGPU(0): #01106442100009e011d007251d01e206e28
(II) AMDGPU(0): #011550006442100001e8c0ad08a20e02d10
(II) AMDGPU(0): #011103e9600064421000018000000000000
(II) AMDGPU(0): #01100000000000000000000000000000072
(II) AMDGPU(0): Printing probed modes for output DisplayPort-1
(II) AMDGPU(0): Modeline "1920x1200"x60.0  154.00  1920 1968 2000 2080  1200 1203 1209 1235 +hsync +vsync (74.0 kHz eP)
(II) AMDGPU(0): Modeline "1920x1080"x60.0  148.50  1920 2008 2052 2200  1080 1084 1089 1125 +hsync +vsync (67.5 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x50.0  148.50  1920 2448 2492 2640  1080 1084 1089 1125 +hsync +vsync (56.2 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x59.9  148.35  1920 2008 2052 2200  1080 1084 1089 1125 +hsync +vsync (67.4 kHz e)
(II) AMDGPU(0): Modeline "1920x1080i"x60.0   74.25  1920 2008 2052 2200  1080 1084 1094 1125 interlace +hsync +vsync (33.8 kHz e)
(II) AMDGPU(0): Modeline "1920x1080i"x50.0   74.25  1920 2448 2492 2640  1080 1084 1094 1125 interlace +hsync +vsync (28.1 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x30.0   74.25  1920 2008 2052 2200  1080 1084 1089 1125 +hsync +vsync (33.8 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x25.0   74.25  1920 2448 2492 2640  1080 1084 1089 1125 +hsync +vsync (28.1 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x24.0   74.25  1920 2558 2602 2750  1080 1084 1089 1125 +hsync +vsync (27.0 kHz e)
(II) AMDGPU(0): Modeline "1920x1080i"x59.9   74.18  1920 2008 2052 2200  1080 1084 1094 1125 interlace +hsync +vsync (33.7 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x30.0   74.18  1920 2008 2052 2200  1080 1084 1089 1125 +hsync +vsync (33.7 kHz e)
(II) AMDGPU(0): Modeline "1920x1080"x24.0   74.18  1920 2558 2602 2750  1080 1084 1089 1125 +hsync +vsync (27.0 kHz e)
(II) AMDGPU(0): Modeline "1600x1200"x60.0  162.00  1600 1664 1856 2160  1200 1201 1204 1250 +hsync +vsync (75.0 kHz e)
(II) AMDGPU(0): Modeline "1280x1024"x75.0  135.00  1280 1296 1440 1688  1024 1025 1028 1066 +hsync +vsync (80.0 kHz e)
(II) AMDGPU(0): Modeline "1280x1024"x60.0  108.00  1280 1328 1440 1688  1024 1025 1028 1066 +hsync +vsync (64.0 kHz e)
(II) AMDGPU(0): Modeline "1152x864"x75.0  108.00  1152 1216 1344 1600  864 865 868 900 +hsync +vsync (67.5 kHz e)
(II) AMDGPU(0): Modeline "1280x720"x60.0   74.25  1280 1390 1430 1650  720 725 730 750 +hsync +vsync (45.0 kHz e)
(II) AMDGPU(0): Modeline "1280x720"x50.0   74.25  1280 1720 1760 1980  720 725 730 750 +hsync +vsync (37.5 kHz e)
(II) AMDGPU(0): Modeline "1280x720"x59.9   74.18  1280 1390 1430 1650  720 725 730 750 +hsync +vsync (45.0 kHz e)
(II) AMDGPU(0): Modeline "1024x768"x75.0   78.75  1024 1040 1136 1312  768 769 772 800 +hsync +vsync (60.0 kHz e)
(II) AMDGPU(0): Modeline "1024x768"x60.0   65.00  1024 1048 1184 1344  768 771 777 806 -hsync -vsync (48.4 kHz e)
(II) AMDGPU(0): Modeline "800x600"x75.0   49.50  800 816 896 1056  600 601 604 625 +hsync +vsync (46.9 kHz e)
(II) AMDGPU(0): Modeline "800x600"x60.3   40.00  800 840 968 1056  600 601 605 628 +hsync +vsync (37.9 kHz e)
(II) AMDGPU(0): Modeline "720x576"x50.0   27.00  720 732 796 864  576 581 586 625 -hsync -vsync (31.2 kHz e)
(II) AMDGPU(0): Modeline "720x576i"x50.0   13.50  720 732 795 864  576 580 586 625 interlace -hsync -vsync (15.6 kHz e)
(II) AMDGPU(0): Modeline "720x480"x60.0   27.03  720 736 798 858  480 489 495 525 -hsync -vsync (31.5 kHz e)
(II) AMDGPU(0): Modeline "720x480"x59.9   27.00  720 736 798 858  480 489 495 525 -hsync -vsync (31.5 kHz e)
(II) AMDGPU(0): Modeline "720x480i"x60.0   13.51  720 739 801 858  480 488 494 525 interlace -hsync -vsync (15.8 kHz e)
(II) AMDGPU(0): Modeline "720x480i"x59.9   13.50  720 739 801 858  480 488 494 525 interlace -hsync -vsync (15.7 kHz e)
(II) AMDGPU(0): Modeline "640x480"x75.0   31.50  640 656 720 840  480 481 484 500 -hsync -vsync (37.5 kHz e)
(II) AMDGPU(0): Modeline "640x480"x60.0   25.20  640 656 752 800  480 490 492 525 -hsync -vsync (31.5 kHz e)
(II) AMDGPU(0): Modeline "640x480"x59.9   25.18  640 656 752 800  480 490 492 525 -hsync -vsync (31.5 kHz e)
(II) AMDGPU(0): Modeline "720x400"x70.1   28.32  720 738 846 900  400 412 414 449 -hsync +vsync (31.5 kHz e)
(II) AMDGPU(0): EDID for output DisplayPort-2
(II) AMDGPU(0): Output eDP connected
(II) AMDGPU(0): Output DisplayPort-0 disconnected
(II) AMDGPU(0): Output DisplayPort-1 connected
(II) AMDGPU(0): Output DisplayPort-2 disconnected
(II) AMDGPU(0): Using spanning desktop for initial modes
(II) AMDGPU(0): Output eDP using initial mode 3840x2160 +0+0
(II) AMDGPU(0): Output DisplayPort-1 using initial mode 1920x1200 +3840+0
(II) AMDGPU(0): mem size init: gart size :bfe3a000 vram size: s:6ee2d000 visible:d6dd000
(==) AMDGPU(0): DPI set to (96, 96)
(==) AMDGPU(0): Using gamma correction (1.0, 1.0, 1.0)
(II) AMDGPU(0): [DRI2] Setup complete
(II) AMDGPU(0): [DRI2]   DRI driver: radeonsi
(II) AMDGPU(0): [DRI2]   VDPAU driver: radeonsi
(II) AMDGPU(0): Front buffer pitch: 23040 bytes
(II) AMDGPU(0): SYNC extension fences enabled
(II) AMDGPU(0): Present extension enabled
(==) AMDGPU(0): DRI3 enabled
(==) AMDGPU(0): Backing store enabled
(II) AMDGPU(0): Direct rendering enabled
(II) AMDGPU(0): Use GLAMOR acceleration.
(II) AMDGPU(0): Acceleration enabled
(==) AMDGPU(0): DPMS enabled
(==) AMDGPU(0): Silken mouse enabled
(II) AMDGPU(0): Set up textured video (glamor)
(II) AMDGPU(0): RandR 1.2 enabled, ignore the following RandR disabled message.
(II) AMDGPU(0): Setting screen physical size to 1524 x 571
Jun 28 08:27:39 frostgiant kernel: [    1.538063] [drm] AMDGPU Display Connectors
Jun 28 08:35:12 frostgiant kernel: [    1.536398] [drm] AMDGPU Display Connectors
Jun 28 08:41:24 frostgiant kernel: [    1.560076] [drm] AMDGPU Display Connectors
Jun 28 08:44:26 frostgiant kernel: [    1.561918] [drm] AMDGPU Display Connectors

Here are screenshots of the display configurations:

First monitor (iMac retina)

Second monitor

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2

I don't know how to resolve this in Ubuntu 17.10. I upgraded to 18.04, and there I could resolve it. As of 18.04, the display settings have more advanced options that allow changing the resolution of the main monitor without making the 2nd one unusable.

Open the Displays setting, and then:

  • Assuming that the iMac is the Built-in display
  • Change Resolution from 3840x2160 (16:9) to 1920x1080 (16:9)
  • Set Scale to 100%

The above changes will trigger a change in the Display Arrangement area, putting a gap in between the two displays, so that they are no longer joined, like this:

displays not joined

Move the displays next to each other so that they are joined, to look like this, and then click Apply:

enter image description here

Note, if you have other running applications in your X session, after this change they may behave strangely, until the end of the session. For example, they may appear zoomed, or they may make the mouse pointer appear zoomed, or the font may look strange even in applications of normal size. To fix all that, logout and login. In the new session, all applications should behave normally.

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