Reproduction steps:
1. Open the attached “MouseButtons” project
2. Open the “SampleScene” scene
3. Enter Play Mode
4. Press the left and right mouse buttons
5. Move the cursor outside the Game view
6. Release the right mouse button
7. Release the left mouse button
8. Observe the Console

Expected result: “KeyUp: Mouse0” was logged
Actual result: “KeyUp: Mouse0” was not logged

Reproduced with: 2020.3.36f1, 2021.3.5f1, 2022.1.8f1, 2022.2.0a18, 2023.1.0a2

Reproduced on: Windows 10, Windows 11

Notes:
1. Could not reproduce in Player
2. Could not reproduce with keyboard keys
3. Also reproducible with the right mouse button
4. KeyDown isn’t called when pressing the left mouse button in Game view for the first time after reproduction (Unity thinks it’s still pressed even though physically it’s not)
5. OnApplicationFocus and OnApplicationPause do not return true during reproduction, indicating that the Game view does not lose focus