Reproduction Steps:
1. Import attached project.
2. In the Menu, select "BugReport" -> "TestCode".
3. Click and hold the middle mouse button in the opened window.
4. Drag mouse pointer outside of the window.
5. Release middle mouse button.

Expected results: "Event.current.rawType == EventType.mouseUp" should be fired, even if mouse position is located outside the EditorWindow.
Actual results: "Event.current.type == EventType.mouseDown" remains fired.

Reproducible with: 2017.1.0b1
Not reproducible with: 5.5.3p1, 5.6.0p1
Regression introduced in: 2017.1.0a4