Steps to repro:

1. Open Unity on OSX;
2. Create new project;
3. Expand Layout dropdown list in the upper right corner and select 'Default';
4. Move Editor to external display;
5. Drag Game view tab to the left side of the Scene View tab, so it would become separate active tab.

Actual result:
Trashy artifacts appear (see attached gif video).

Regression introduced due to refactoring of GfxWindow in platform/osx/editor/GfxDeviceWindow branch.

Expected result:
Tabs should be dragged without trashy artifacts.

Notes:
- Reproducible in platform/osx/editor/GfxDeviceWindow branch;
- Doesn't repro in 5.5.0b3;
- Reproducible only on OSX 10.11.6; not reproducible on Windows 10;
- Also reproducible when dragging Scene view and Asset Store tab;
- Seems to repro only on Apple devices with Intel Iris GPU;
- The branch code is on the way to trunk; should be fixed until 5.5. ships.