I recently installed Mint 17.1 KDE 64-bit on an HP Stream 11 laptop. When I close the laptop lid, the screen will not lock. This happens when I set the laptop to sleep when the lid is closed and "Lock screen on resume" in Power Management and also when I have it set to simply lock screen when the lid closes. Screen locking itself works. When I, for example, use CTRL+ALT+L, right click on the desktop or choose it as an option from "Leave" it locks fine. I just can't get it to lock when the lid is closed. I have tried all of the screen locker types under Screen Locker and none of them work. For a short period of time I believed I had solved the issue by selecting "Require password after" and choosing 1 second under Screen Locker, but by the next day that stopped working for some reason. I have two other laptops (different brands/models) and they work fine. I have rated the severity of this bug as Major since I believe that it is a security issue. Reproducible: Always Steps to Reproduce: 1. Close laptop. 2. Wait a while. 3. Open laptop. Actual Results: The screen isn't locked. Expected Results: The screen should lock when the lid is closed and it is put to sleep.
In case it is helpful, I have also noticed that the majority of the time when I close the lid and then re-open it, not only is the screen not locked, but it appears as if a right-click has been registered. Usually the menu associated with a right-click will appear wherever the mouse happens to be.
The screen locker will now use logind to detect an imminent suspending and delay it until it has made sure the screen is locked.
That MAY solve the problem, however, notice that the problem also exists when the laptop is set to simply lock the screen (not sleep/suspend) when the lid is closed. From your description, it doesn't sound like it will resolve the problem in this situation. Also, being fixed in 5.2 doesn't help those of us on 4.13. Any chance it can be fixed in 4.13?
I've come up with a workaround for this problem for those of us not using 5.2.0. It isn't perfect, but it works. I have described it at: http://forums.linuxmint.com/viewtopic.php?f=49&t=189901&p=984408#p984408