To reproduce:
1. Create a HDRP template
2. Open a Game View window
3. In the Hierarchy window select any object in the scene and select back to Main Camera
4. Observe the black outline appear and disappear in Game View

Expected: selecting a camera should not cause appearance of black outline around game objects
Actual: selecting a camera causes appearance of black outline around game objects

Reproduced with: 2018.4.0f1, 2019.1.5f1, 2019.2.0b4, 2019.3.0a4

Notes:
- Could not reproduce with 2017.4 since HDRP does not exist for the Editor version
- Outline intensity depends on Anti-Aliasing mode set on the Camera (Temporal Anti-Aliasing)
- Brightness in the Game View changes when Anti-Aliasing is off, and cursor hovers between Game and other Editor windows
- Issue does not reproduce just with Post Processing package, in a non-HDRP project