Up until I switched to El Capitan (from Mavericks) I was able to press the key corresponding to the start of the user name I wanted to log into at the post-boot login screen. This would select that user, and I could then start typing in the password. No mousing needed. After the switch, I found that I occasionally needed to move the mouse from the upper-left corner where it was hiding (a long way on a 27" iMac) and select the user icon before the keyboard input was recognized. I then learned that just clicking on the mouse wherever the cursor was located was enough to trigger keyboard recognition. As time has progressed, this has gotten worse to the point where the keyboard is now almost always ignored until I click on the user icon I want to log in as. I'm on 10.11.5, a 2011 iMac, and the keyboard is a Kinesis Freestyle 2 for Mac with a USB interface plugged directly into the back of the iMac (not through an external USB hub). I also have 10.11.5 on a 2013 MBA, and I have never had this problem. (As an aside, it's interesting that the screen sequence at boot-up is different on the MBA than on the iMac, both with 10.11.5 and both with two users.) Anyone found a cure for this?