I have a DELL XPS 9570 laptop with xubuntu (18.04.1 LTS) installed.
My problem is that when I have the lid closed with external display connected, after I lock the screen I can't unlock it without opening a lid. So what I do now, every time I need to unlock the screen I open the lid, screen wakes up, I login and close the lid.
I'm also not fully sure if its a good thing or not but after I lock the screen, display goes black (but doesn't turn off, just like it would display black image). When I use mouse or keyboard, it wakes up and shows the greeter. Still of course only if lid is opened.