I have a Macbook Pro running Mojave 10.14.4 with an external montior, I have set up mirrored display and made the external monitor my primary display.
When I close the lid the monitor goes black, as expected, when I press key to get it to light up again all the applications I opened while the lid was opened are not shown on the external monitor. If I open up the lid the monitors go black and I press a key I can see all applications on both monitors. As I hope to just come in, plug in my Macbook and see what I'm working on in my primary monitor it would be nice if I did not have to open up my secondary monitor to do so.