Scene View elements become very bright after enabling HDR if the Color Space is set to 'Linear'.

Steps to reproduce:
1) Open the attached project.
2) Press on the GUI button in Scene View.

The button toggles HDR On/Off on the Main Camera. When HDR is enabled, the button itself and any other elements that are visible through Scene camera become washed out. If Color Space is set to 'Gamma' this doesn't occur. Game View doesn't have this issue.
This happens with OpenGL and DirectX. For Metal on macOS enabling the experimental 'Metal Editor Support' exposes the issue with both HDR enabled and disabled if Linear Color Space is used.

Reproduced on:
5.6.0b7, 5.5.1p3, 5.4.4p3
2017.2.0b5, 2017.1.0p2, 5.6.2p4

Did not reproduce:
5.3.7p4

Tested on:
macOS 10.12.3
Windows 7