This doesn’t happen every time, just often. If I turn the monitor off and back on again the log in screen is fine. It always fine when I boot up. It just happens when I log out of the desktop. I recently changed the graphics card but the problem has only shown up over the last few days. The new card is considerably quicker than the previous one.
The monitor is a dell u2713um with a native resolution of 2560x1440. The card is GTX 1050 Ti using the nvidia driver.
When the problem occurs the monitor ask for the resolution to be switched to 2560x1440. It has been so it looks like something strange is happening between the transition from the desktop to the log in screen. Sometimes it’s ok sometimes it isn’t.