This issue has been addressed many, many times. I recommend that you search for one of the numerous threads on the topic.
I have yet to come across an answer to this question that actually solves the problem.
If you're not running the .5 update for MacOS, you should. 10.10.5 and 10.11.5 both have significant bug fixes that affect After Effects.
For many folks, using the new CC 2015.3 version (AE version 13.8 - basically CC 2016) solves the problem. It works great for me on my Mac. However, I'm not using a 4K or 5K screen with it. That seems to be the cause of most of the remaining issues.
For those folks, putting a panel underneath the timeline panel (such as the Libraries panel) might do it, while others have found that shrinking the vertical size of the timeline panel works.
Alternatively, making the comp panel full screen during previews also works (either through the checkbox in the preview panel or simply pressing ` with the mouse over the composition panel to full-screen it).