System Information
OS X Yosemite 10.10.2
AMD Radeon R9 M295X 4096MB

Blender Version
Broken: 2.76b f337fea

In the NLA Editor the 'Add Modifier' button at the bottom of the Properties Panel is the wrong size on Retina Display Macs.

Appears to be the same type of bug as in this report, just in a different editor.
Probably no troubleshooting required for this one.

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

1. Add an object and assign some animation keyframes to it
2. Open the NLA Editor
3. Push the action down as a new NLA strip
4. Open the Properties Panel and scroll down to see the button