How to use keyboard with a broken screen
I need to be able to use the laptop keyboard to log in when it’s in clam shell mode.
I have a broken computer attached to a monitor but I can’t get the log in to show up on the monitor.
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If you have a list of users for the login screen, then you can try logging in blindly. After booting the laptop & giving it time to get to the login screen, type the first character of one of the macOS user names followed by pressing the "Return" key. Now enter the password for that user and press the "Return" key again to submit the password. If you happen to have more than one user beginning with the same letter, then you will have to try each one's password since I don't know how to tell which one it would select.
Even this is not guaranteed to help since macOS is very quirky when it comes to wanting to display everything on the built-in display whether it is working or not. If a macOS update was installed, then macOS may never display anything on the external display until you get past the pop-up windows on the main screen since those windows cannot even be moved to the external screen. Sometimes toggling display mirroring on/off won't work until you are logged into macOS.
If you can log in, but still not get a mirrored display, but you can get an extended screen onto the external display, then you can try launching the System Preferences/Settings either from the Dock if you are lucky or from the Apple menu. Most likely it will open on the broken built-in display, but if you Right-click or Control-click the System Preference/Setting app icon on the Dock, then you can select "Displays" and if lucky, you will get a Display window appear on the external display.....select "Group Windows" or something similar to hopefully bring the main System Preference/Setting window to the external display where you can enable mirroring. Maybe the Right-click, or Control-click on the System Preference/Settings icon on the Dock will give you an option to mirror Displays. Or if you have the menubar Display options enabled, then you can use the menu bar option to configure the displays.
Sooner or later you will encounter a situation where you won't be able to interact with macOS until you deal with whatever is shown only on the broken built-in display. It is so bad, that even on some of the more recent Apple laptops even if the built-in Display is completely disconnected & the external display is the only display present....some things will insist on showing only on the built-in Display.....yes, that has happened to me while trying to support my organization's Macs. So be prepared if you don't have this broken display repaired.
FYI, if the display just has a bad backlight, then you have two things you can try. One is closing the lid of the display, then very slowly & carefully lift it up just a little bit to see if you notice the backlight light up & see the video on the screen. Sometimes you may be able to lift the lid to about 45 degrees before the backlight goes out if the internal cable is bad, but it will be enough for you to log in & navigate to turn on display mirroring.
Second, if it is a backlight issue and the previous suggestion does not help, then try using the laptop in a dark room with a flashlight to shine onto the screen which may allow you to see a faint image on the built-in screen so that you can log in & toggle display mirroring on. It will be hard to see the pointer, but it is possible with a bit of practice. I like to move the pointer diagonally so it it placed into the upper left corner so I can focus on seeing the pointer there & trying to move it across the top of the menubar until I have to move it down the screen.
