How to reproduce:
1. Open the attached project (1252149.zip)
2. Open the "Main" scene
3. Select the "Main Camera" GameObject from the Hierarchy
4. Enter Play Mode
5. Move the cursor to the right side of the Game window
6. In the Inspector observe the "Mouse Pos" and "Screen Size" values on the "MsTest" script

Expected result: The "Mouse Pos" X value is almost the same as the "Screen Size" X value
Actual result: The "Mouse Pos" X value is approximately equal to half of the "Screen Size" X value

Reproducible with: 2019.3.16f1, 2019.4.0f1 (0.2.10), 2020.1.0b12, 2020.2.0a14 (1.0.0)
Not reproducible with: 2018.4.23f1 (0.2.1-preview)

Notes:
1. Tested on Windows 10 and Ubuntu 18.04 (Doesn't reproduce on Windows 10)
2. Doesn't Reproduce with the old Input System
3. Reproduces in the Editor, but not the Player