The only time that I've seen this happen is when I click on the "x" on the side menu on the left hand side to get more real estate. After closing the side menu, if you start navigating on the main frame, then that is okay. The issue comes in when you click on "Contents" to again display the side menu. When you click on this, the "main frame" focus is now located in the side menu. Unfortunately, it appears like a browser refresh can restore the state back to the original state.
I tried the same steps with IE, and IE appears to be working as design. I'll submit this issue to our documentation group for further investigation.