Steps to reproduce:

1. Open Unity in GlCore mode on OSX;
2. Create new project;
3. Resize one of the Editor tabs (Scene view/Game view/Hierarchy/Inspector).

Actual result:
Resizing Editor tabs leaves white traces (see attached gif video). On the video traces are very big, though really they are smaller.

Regression introduced in Unity 5.4.

Expected result:
Resizing Editor tabs should not leave white traces. Expected behaviour could be found in 5.3.1f1.

Notes:
- Reproduces in 5.4.0b4, 5.4.0b3 for OSX 10.11.3;
- Doesn't reproduce in 5.3.1f1 and earlier;
- Reproduces in OpenGL 2.1 mode for OSX 10.11.3;
- Also reproduces on Windows 10 in GlCore mode, however that behaviour also is in 5.3 and 5.2 (isn't regression);
- Doesn't reproduce in DX11 mode.