Steps to repro:

1. Open Unity;
2. Create new project;
3. Drag any rendering tab (Scene or Game view) so it is fully detached;
4. Observe tab's preview.

Actual result:
Preview of the rendering tab being dragged is not shown (see attached actual.png screenshot).
Regression introduced in Unity 5.5.

Expected result:
Preview of the draggable rendering tab should be shown (see attached expected.png screenshot).

Notes:
- Reproducible in 5.6.0a2, 5.5.0.b11, 5.5.0b10;
- Not reproducible in 5.4.2p3;
- Reproducible on both Windows 10 and OSX 10.12.1.