Repro steps:

1. Open Unity (empty or existing project)
2. Undock any panel (tested with Console)
3. To help visualise issue, move undocked panel so its top part covers half of the play/pause/next frame buttons. (see user-provided screenshots)
4. Notice how the titlebar of the undocked panel blends in with the Unity background.
(5). If undocked panel is still casting a drop shadow, close the panel and reopen it through the Window menu (i.e. Window -> Console or using Shift+Cmd+C on Mac).

Expected: panels should cast drop shadow

Notes:
- Resizing panel when drop shadow is missing makes the shadow appear.
- Closing panel and reopening it from the Window menu makes the shadow disappear.
- Could not reproduce issue with OS X 10.10. Could repro after upgrade to 10.11.2 (El Capitan).
- Reproduced with Unity v. 5.2.3f1 and 5.3.0f4
- Using MacBook Pro (Retina, 15-inch, Mid 2015)