Steps to reproduce:

1. Open attached project
2. Go to the project folder and delete CS project files
3. In editor, make sure that code editor is set to built-in MonoDevelop
4. Double-click on "TestDos" script and in console notice message which is logged because of OnGeneratedCSProjectFiles callback
5. Close MonoDevelop, clear Unity console and go to project folder once again
6. Delete CS project files
7. In editor, change code editor to Visual Studio (VS2015 for example)
8. Double-click on "TestDos" script again but this time no callback is triggered

Reproduced with: 5.2.4f1, 5.3.4p4, 5.4.0b15