我在 iMac Retina 上运行 Ubuntu 17.10（更新：现已升级至 18.04，没有任何变化），使用戴尔作为第二台显示器。iMac（第一台显示器）的默认分辨率为 3840x2160（16:9）。此时，所有内容都太小，无法读取，但至少我可以看到第二台显示器（分辨率为 1920x1200（16:10））。
当我将 iMac 的分辨率更改为 1920x1080 (16:9) 时，第二台显示器变为空白。位于其上的窗口仍然存在（不可见），我可以将鼠标移到那里（不可见）。
我用所有设置 | 显示更改设置。更改后，系统会询问是否保留设置，无论我如何回答，第二台显示器都不会恢复。重启后也无法使用。
我不知道这是否相关，但我在非视网膜 iMac 上运行的外部 SSD 磁盘上安装了 Ubuntu，后来我将 iMac 换成了视网膜。除了显示分辨率之外，一切似乎都运行良好（到目前为止）。
我该如何修复或调试这个问题？
其中有些内容/var/log/syslog可能相关：
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
线路匹配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,
线路匹配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
以下是显示配置的屏幕截图：
答案1
我不知道如何在 Ubuntu 17.10 中解决这个问题。我升级到了 18.04，然后就可以解决这个问题了。从 18.04 开始，显示设置有更多高级选项，允许更改主显示器的分辨率，而不会使第二台显示器无法使用。
打开显示器设置，然后：
- 假设 iMac 是内置显示屏
- 改变解决从 3840x2160 (16:9) 到 1920x1080 (16:9)
- 放规模至 100%
上述变化将引发陈列布置区域，在两个显示器之间留出空隙，这样它们就不再连接在一起了，就像这样：
将显示移动到彼此相邻的位置，使它们连接在一起，看起来像这样，然后单击申请：
请注意，如果您的 X 会话中还有其他正在运行的应用程序，则在进行此更改后，它们可能会表现异常，直到会话结束。例如，它们可能会显示放大，或者它们可能会使鼠标指针显示放大，或者即使在正常大小的应用程序中字体也可能看起来很奇怪。要解决所有这些问题，请注销并登录。在新会话中，所有应用程序都应该正常运行。