Steps to repro:

1. Open Unity on OSX;
2. Create new project;
3. Add terrain gameobject (Gameobject -> 3D Object -> Terrain);
4. Go to any tabs of Inspector's Terrain component having brush icons (e.g. Raise/Lower Terrain);
5. Move the brush over the terrain - notice that brush cursor changes position immediately after mouse cursor positions changes;
6. Move the cursor to the external display;
7. Return back to the initial display;
8. Move the brush over terrain.

Actual result:
Brush cursor position is updating with delay (~1 second). See attached gif video.

Expected result:
Brush cursor position should always change its position immediately after mouse cursor position's change.

Workaround is to click anywhere in the Editor at the initial display.

Notes:
- Reproduces in 5.4.0b5, 5.4.0b4, 5.3.1f1.
- Reproduces only on OSX 10.11.3 (checked). Doesn't repro on Windows 8;