No Intellisense for Unity Assembly-CSharp scripts when working in Assembly-CSharp-Editor

I am having an issue with Unity, where Visual Studio 2017 will not recognize classes in the standard Assembly-CSharp project when working on custom editor scripts in Assembly-CSharp-Editor.

Everything compiles correctly in Unity and functions as expected, however I get no IntelliSense for MonoBehaviour classes when working in Editor scripts, and Visual Studio gives me "type or namespace could not be found" errors as shown:

The solution layout can be seen here, I don't think I've done anything odd:

I've tried re-importing everything into Unity, deleting the solution and project files so that Unity will recreate them, but no joy.

Is there something obvious I am missing or does anyone have any insight? Thanks.