Comment by novirium

Highly specific to Dell XPS laptops, but I used to have a similar issue to this. Eventually realised that the display wasn't actually turning off sometimes when I closed the lid. After much mucking around, it eventually turned out that in some of these laptops the magnet in the lid that triggers the "lid closed" action can become physically unstuck, and move around in the display housing (on my XPS 9350 this is in the bottom right corner of the display). Using an external magnet to jiggle it around both allowed testing to trigger the sensor and confirm the issue, and also to move it back to its proper location. Seems to work properly again now.