How to reproduce:
1. Open the attached project (1250718.zip)
2. Press Ctrl+B on the keyboard and select a build path
3. Once the build is complete move the mouse cursor on the button in the middle of the Player window
4. Click on the button to make sure that the Player window is focused
5. Notice how the button turns red when highlighted
6. Move the mouse cursor straight down from the center of the button until it is outside the Player window while observing the button

Expected result: The button is unaffected
Actual result: The button becomes highlighted

Reproducible with: 2018.4.12f1, 2018.4.23f1, 2019.3.16f1, 2019.4.0f1, 2020.1.0b11, 2020.2.0a13
Not reproducible with: 2018.4.11f1

Notes:
1. Tested on Windows 10, Windows 7 and Ubuntu 18.04 (Doesn't reproduce on Ubuntu 18.04)
2. Doesn't reproduce on 3840 × 2160 resolution displays, only on 1920 x 1080 and 1366 x 768
3. There is an additional project (1250718_2018.4.12.zip) for 2018.4 versions because downgrading the (1250718.zip) reproduction project can break it