It's a by design behavior after https://ono.unity3d.com/unity/unity/pull-request/82141/_/editor/fix/1115348-osx-windows-infront-other-apps
To help reduce the pain we should implement something like "stick on top" toggle for detached windows

Steps to repro:
1. Open attached project
2. Edit -> Project settings - Graphics
3. Try to drag RenderPipelineAsster from project window to "SRP Settings" option in Render Settings.

Expected result: Project settings window stays on top, so user I could drag RenderPipeline asset to its window.
Actual result: Project Settings window gets hidden behind the Unity Editor window, ruining single display experience.

Reproducible with: 2018.3.14f1, 2019.1.0b5, 2019.1.0f2, 2019.2.0a5, 2019.2.0a13
Not Reproducible with: 2017.4.26f1, 2018.3.9f1, 2019.1.0b4, 2019.2.0a4

Note: Does not reproduce in Windows 10.