Reproduction steps:
# Open the user attached project
# Press File → Build And Run
# Scale down the Player window horizontally
# Press ALT + ENTER

Expected result: The player window resizes to fill the whole screen, no black bars
Actual result: The player window does not resize to fill the whole screen, black bars

Reproducible with: 2020.3.39f1, 2021.1.28f1, 2021.2.0b9
Not reproducible with: 2021.2.0b10, 2021.3.9f1, 2022.1.14f1, 2022.2.0b6, 2023.1.0a8

Reproduced on: Windows 10

Additional info:

This issue was [addressed in 2021.20b10 release|https://unity.com/releases/editor/beta/2021.2.0b10] as an "improvement" (not a customer reported bug):

Windows: Alt + Enter now defaults to native resolution rather than the resolution that the window was in windowed mode. This makes the image more crisp and reduces the chance of letterboxing.