I've found another issue that may be related to this. (Also, I think I already posted this here, but I can't find it anywhere. So forgive me, if I repost).
When I check input.isKeyDown on Shift or Ctrl keys, as soon as I press the key once, the isKeyDown method always returns true. This happens against the latest version of the dev branch. I verified the problem on a friends slick setup and he also could check, that the bug did not occur in the "stable" version published on the slick homepage.
This happens even if the window does not lose focus. I'm not sure which keys are all affected by this, I only found LCTRL, RCTRL, LSHIFT and RSHIFT are not working up until now.
Here is the test class I used to verify the problem outside of my project:
http://pastebin.com/NUBZjAWK