It seems as though editor windows will sometimes respond to mouse events outside the window area. If you drag an editor window, then place your cursor anywhere else in the editor and quickly click and drag the mouse, the editor window will respond to the drag and will move. It's possible to lose editor windows completely offscreen by doing this.

1. Open Unity
2. Open the attached project
3. Open any editor window and make sure it is undocked
4. Drag the editor window anywhere
5. After releasing the drag, position your cursor outside the editor window, in the project view
6. click and drag the mouse

Expected Result:
The editor window loses focus and the project window gains focus.

Actual Result:
The editor window will respond to the mouse drag and will move across the screen.

Tested on Windows.

Occurs on 2019.2.0a11

Does not occur on 2019.2.0a10