I have a dual monitor setup with an iMac. The secondary display is connected through a thunderbolt to HDMI cable. I just realized that the hand tool works fine when the timeline is on the iMac, but doesn't work when it on the secondary display. Odd...
This is a known bug with After Effects CC 2015 (13.5) on Mac OS. The hand tool will not pan left or right in the Timeline panel when that panel is on a second monitor that is to the LEFT of the primary monitor.
The problem doesn't occur if you move the Timeline panel to the primary monitor; or in System Preferences > Display reassign the secondary monitor to be the primary or move the secondary monitor arrangement above, below, or right of the primary monitor.
Apologies for the bug. It's an odd one indeed.