[2D] Sprite Editor Slice Type Dropdown is opening behind the Sprite Editor.

In Sprite Editor, Slice Type dropdown disappears if the mouse is not held also Slice Type dropdown
is opening behind the sprite editor after clicking on Type dropdown.

Steps to reproduce :

1. Create a new project.
2. Import 2D sprite
3. Open Sprite, make sure the mode is set to Multiple and open Sprite Editor
4. Click on Slice and try to change the type and notice that the dropdown disappears if the mouse left click is not held.
5. Click on 'Grid by Cell Size'
6. Again click on 'Grid by Cell Size' and notice that the dropdown is opened behind the sprite editor

Expected results: In Sprite Editor, Slice Type dropdown appears after clicking on Type dropdown.

Actual results: In Sprite Editor, Slice Type dropdown disappears if the mouse is not held also it
is opening behind the sprite editor after clicking on Type dropdown.

Reproducible with: 2020.1.0a15 (c34b21e125d7)
Working fine with: 2020.1.0a14, 2019.3.0f1.

Environment: macOS 10.14 only.