I have my Retina Macbook Pro's built-in display set to the scaled resolution "more space" in System Preferences. Whenever I connect to an external monitor (extended mode), the resolution resets to "default". I have to manually change it back to a larger resolution. Is there anyway to lock it on the correct resolution and prevent this behavior?
MacOS – Built-in display resolution resets when connecting to external monitor
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In mirroring mode, both screens need to use the same resolution. The external display probably can't support the huge 2880x1800, so it's lowering yours to match.
In System Preferences > Displays > Arrangement tab, uncheck the mirroring option. Then you should see two separate preferences windows, allowing you to set different resolutions for each display. Note — the Arrangement tab is only visible when you have an external display connected.
I have found the MacBooks to be rather flakey when it comes to multiple monitors. However, I have found that if you hit the option key while selecting the scale radio button you get more options for the 4k monitors.
Also unplugging the HDMI cable seemed to reset the Mac to recognize that the monitor supported more resolutions.
Best Answer
I have got the same problem. It is not that big of a deal to change it everytime, but it still is pretty annoying.
Sometimes it doesn't happen, but most of the times it does.
This morning when I turned on my macbook, it started in my "more space" resolution. When I connected my second screen, display mirroring was turned on (something I never use and don't want). When I turned off display mirroring, my Macbook display went to the default resolution. I changed it back to "more space". Then I unplugged the second monitor. After my Macbook's screen went black, it returned in the default resolution again. I changed it back to "more space". Then I replugged my second monitor and this time it remembered my "more space" setting. I then unplugged the second monitor again and again it remembered by "more space" setting.
It seems as though it remembers the settings until I completely shut down my Macbook. Then, it reverts to the default settings (and sometimes randomly turns on display mirroring). I have to test this thesis more to see if I am correct.