Steps to repro:

1. Open Unity on OSX;
2. Create new project;
3. Open Asset Store (Window -> Asset Store);
4. Select any relatively big project in the Asset Store (in my case 'The Wasteland') and press 'Import' button;
5. While asset import try to drag and detach any tab (in my case, Game View) in Editor.

Actual result:
- Sometimes Unity crashes. Last calls in the stacktrace:

#0 0x000001025706e6 in ContextGLES::Acquire()
#1 0x000001025acaaf in GfxDeviceClient::AcquireThreadOwnership()
#2 0x000001026236c9 in ActivateGraphicsContext(ObjectHandle<GraphicsContext_Tag, void>, bool, int)
#3 0x00000101d11278 in GUIView::BeginCurrentContext()
#4 0x000001024df54f in HandleUtility_CUSTOM_INTERNAL_CALL_Internal_PickClosestGO(ReadOnlyScriptingObjectOfType<Camera>, int, Vector2fIcall const&, MonoArray, MonoArray, int)
#5 0x0000011af02da9 in (wrapper managed-to-native) UnityEditor.HandleUtility:INTERNAL_CALL_Internal_PickClosestGO (UnityEngine.Camera,int,UnityEngine.Vector2&,UnityEngine.GameObject[],UnityEngine.GameObject[],int&) + 0xc9 (0x11af02ce0 0x11af02dd8) [0x117501660 - Unity Child Domain]

- White detached tab's preview window is displayed during asset import (see attached actual.png screenshot).

See attached gif video for clarity.

Inconsistent behaviour between product - since you can't move any tab or do anything with Editor's layout On Windows (DX11).

Expected result:
There should be no ability to change Editor's layout/drag any tabs while importing assets.

Notes:
- Reproducible in: 5.4.0b13, 5.4.0b12, 5.3.4p1, 5.3.3f1, 5.3.2f1, 5.2.2p4;
- Reproducible only on OSX (10.11.3 checked);
- Not reproducible on Windows 10 (DX11 and GlCore mode);
- Crash is not reproducible always (yet white preview of the detached tab is 100% reproable). Here's what may increase crash probability:
1) Detach Game and then Scene when importing assets;
2) Click on white detached tab's white preview window when importing assets