Steps to repro:

1. Open Unity on Windows;
2. Create new project;
3. Detach any tab (i.e. Scene or Game view);
4. Enter Play mode;
5. Move tabs by left clicking on the black zone from the right side of tab's name and drag the tab around;
6. Observe the console.

Actual result:

Errors:
ArgumentException: Getting control 2's position in a group with only 2 controls when doing mouseDrag

are spammed in the console.

Regression introduced in Unity 5.4.0b22.

Expected result:
Errors should not be shown when dragging tabs in Play mode.

Notes:
- Reproducible in 5.4.0b22;
- Not reproducible in 5.4.0b21;
- Reproducible only on Windows 10 and Windows 8.1;
- Not reproducible on OSX;
- Not reproducible when dragging tabs without entering Play mode;
- Not reproducible when detaching tabs in Play mode;
- Not reproducible when dragging tabs by clicking on tab's name.