How can I fix underscan/overscan when the slider isn't shown in System Preferences?

Solution 1:

/var/db/.com.apple.iokit.graphics is the configuration file for displays on macos. The entries are not at all labeled, but in my experience a new monitor will be on the bottom of the list. Each entry is structured as follows:

    <key>IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@2/display0/AppleDisplay-6b3-27ae</key>
    <dict>
        <key>cyuv</key>
        <integer>268435456</integer>
        <key>pscn</key>
        <integer>10000</integer>
        <key>startup-timing</key>
        <data>
        AAAAAAAAAIAJMACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
        AAAAgP4hCgAAAACA/iEKAAAAAID+IQoAAAAAgAcAAGoAAAAIAAAAIAAAADgE
        AAAnAAAAGQAAAAgAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAAA
        AAIAAAAAAAAAAgACAIAAAQAAAAAAAAAAAAAAAAAAAAAA
        </data>
        <key>version</key>
        <integer>2</integer>
    </dict>

The pcsn key and following integer are overscan. My first monitor had some value besides 10000, and changing the value to 10000 fixed it. I tried this with my second monitor and it didn't work, so I just deleted the last few entries. Either method requires a restart.