Steps to repro:

1. Open Unity 5.4 on OSX;
2. Create new project;
3. Detach any tab (e.g. Inspector) from Editor layout by holding left mouse button on tab and moving the cursor to some other area;
4. Move detached tab by grabbing black zone between grey tab name and window icons (close/maximize etc), holding left mouse button and changing cursor position;

5. Notice that the tab is moved slightly along with the cursor;
6. Close Unity;
7. Reopen Unity and recent project;
8. Repeat step 4.

Actual result:
Detached tab is not moved slightly with the cursor after reopening project with changed layout.
See attached video for clarity.
Regression introduced in Unity 5.4.
Inconsistent within product, because when firstly detaching tab and dragging it with mouse - it moves slightly.

Expected result:
Detached tab should always be moved slightly with the cursor, even after reopening the project.
Expected behaviour could be found in Unity 5.3.3f1.

Workaround is to attach the tab back to the current layout, reopen Editor, detach tab again - in this case tab will redraw again when dragging it.

Notes:
- Reproducible in: 5.4.0b8, 5.4.0b7, 5.4.0b6;
- Not reproducible in: 5.3.3f1, 5.3.2f1;
- Reproducible only on OSX (10.11.3 tested) in GlCore mode;
- Not reproducible on Windows 10.