Reproduction steps:

# Create an empty project
# Build the project
# Set Windows Project setting to “PC screen only” (Win + P shortcut)
# Run the Player
# Observe the Player’s resolution
# Exit the Player and set Windows Project setting to “Second screen only” (Win + P shortcut)
# Run the Player again
# Observe the Player’s resolution

Expected result: no black bars should be seen
Actual result: black bars are present

Reproducible with: 2021.2.0a16, 2021.2.19f1, 2021.3.5f1, 2022.1.7f1, 2022.2.0a18, 2023.1.0a2
Not reproducible with: 2019.4.39f1, 2020.3.36f1, 2021.2.0a15

Reproducible on: Windows 10 (21H2)

Notes:

* One of the two monitors has to be set as a vertical display
* Deleting the HKEY_CURRENT_USER\\SOFTWARE<company_name><product_name> registry values before opening the Player on another display fixes the issue