Letterbox fills with artifacts from Steam Overlay when the game aspect ratio is different that in the current settings of the display.

1. Log into Steam and have it running in the background
2. Open the attached project
3. Open scene 'main' and Play scene
Note: Your Steam username should be Logged in the console
4. Stop playback
5. Build & Run
6. Toggle 'Windowed' mode off
7. Select a resolution with an aspect ratio that's different than in your current display settings
8. Click 'Play'
8. Open and close the Steam overlay with 'Shift+Tab'

Result: The black bars or 'letterbox' has leftover artifacts from the Steam Overlay