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I'm trying to connect a Dell 2208WFPT Flat Monitor to my MacBook Air (macOS Mojave, 10.14.6) using Thunderbolt. The symptoms are as follows : on the monitor, I only see the main background image and the upper menu bar of my Mac, and nothing else. In System Preferences > Display, the external monitor is not detected.

The specification for my MacBook Air says :

Video Support Simultaneously supports full native resolution on the built-in display and up to 3840-by-2160 resolution at 60Hz on an external display, both at millions of colors

Thunderbolt digital video output Native Mini DisplayPort output VGA, HDMI, DVI, and dual-link DVI output using adapters (sold separately)

I browsed https://support.apple.com/en-us/HT201177 and the related questions but found nothing useful so far.

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When you see your background image and the upper menu bar on the external monitor, it is connected and working as an extended display.

You can grab any window on screen and move it to the external monitor by dragging it "over the edge" of your main monitor. Depending on your setup that is either the left or the right edge of the main monitor.

If you rather want the new monitor to mirror your internal display, you can open up System Preferences, select Displays and then press Display Settings, choose the Dell monitor and for Use as: select Mirror for ....

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  • Thanks for your feedback. When I push a window to the right, it starts getting displayed on the external monitor indeed, but the new window vanishes as soon as my finger leaves the mouse (I used the mouse to draw the window to the right).
    – Ewan Delanoy
    Feb 15 at 9:34
  • And now for the obvious question : with my PC, the working mechanism (which is far more convenient and user-friendly than what you describe) is that the display on the native monitor is identical to the one on the external monitor. Is that achievable with a Mac also ?
    – Ewan Delanoy
    Feb 15 at 9:35
  • Yes, that is achievable on a Mac as well. I have added to the answer how to make that happen. I do disagree with you however that such a setting is "more convenient and user-friendly" in the general case - it might be in some case, but for most people adding extra monitors to their laptop, they want to extend the desktop. Mirroring are probably most often used when connecting TVs, projectors and the like.
    – jksoegaard
    Feb 15 at 9:39
  • If the window vanishes as soon as your finger has left the mouse, you have not dragged it enough. Drag the window fully over to the other monitor, and it stays on that monitor.
    – jksoegaard
    Feb 15 at 9:40
  • All your suggestions seem to work, answer accepted. Thx
    – Ewan Delanoy
    Feb 15 at 9:51

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