How to reproduce:
1. Install Rider (JetBrains) IDE
2. Enable Auto-save functionality
3. Modify a script
4. Alt+tab from Rider to Unity
5. Notice the autosaved changes are not picked up (Editor not recompiling)

Expected behaviour: Unity picks up changed files when regaining focus

Reproduced on: 5.3, 5.4, 5.5, 5.6, 2017.1

Won't Fix reason: Rider is too slow.
When Unity gets focus we expect all the file to have the correct states. The fact that rider has not finished saving means that it is just too slow.