Steps to reproduce:
1. Open user attached project
2. Change render mode to Screen Space - Camera
3. Assign a main camera
4. Add a new sorting layer below the Default one
5. Choose the new sorting layer for the "Canvas" game object
6. Play the scene, click on the dropdown
7. Notice that that the dropdown list appears behind of the object

Expected result: Dropdown list should copy the parent canvas layer when the panel is opened

Reproduced in: 2018.1.0a1, 2017.3.0b4, 2017.2.0f3, 2017.1.2p1, 5.6.4f1