How to reproduce:
1. Open user-submitted project (Bug Report IMGUI)
2. From the toolbar select BUG REPORT > Bug Test Window
3. Click a couple of times inside of the window to get the current mouse position
4. Click on the first Enum 0 popup field and click away from it
5. Click on the second Enum 0 popup field and click away from it

Expected result: Event.current.mousePosition returns the correct value in both cases
Actual result: Event.current.mousePosition returns the wrong value in both cases

Reproducible with(Windows): 2018.3.0a6, 2018.4.1f1, 2019.1.5f1, 2019.2.0b5, 2019.3.0a4
Not reproducible with(Windows): 2017.4.28f1, 2018.3.0a5

Reproducible with(macOS): 2017.4.28f1, 2018.4.1f1, 2019.1.5f1, 2019.2.0b5, 2019.3.0a4

Notes:
The first popup field is drawn before returning mouse position. Clicking away from it does not return a new mouse position
The second popup field is drawn after returning mouse position. Clicking away from it returns the wrong mouse position(as if the mouse was not moved from the field)
This issue is a regression for Windows, but not a regression for macOS