Unity Editor global hotkeys as of 2018.1

globalEventHandler.cs

[InitializeOnLoad]
public static class StaticEvents {
  static StaticEvents() {
    FieldInfo globalEventHandler = typeof(EditorApplication).GetField("globalEventHandler", BindingFlags.Static | BindingFlags.NonPublic);
    var value = (EditorApplication.CallbackFunction)globalEventHandler.GetValue(null);

    value += () => {
      // Triggers for EVERYTHING
      var e = Event.current;

      if (e.type == EventType.KeyDown) {
        if (e.keyCode == KeyCode.LeftShift || e.keyCode == KeyCode.RightShift) {
          Debug.Log("SHIFT DOWN EVERYBODY!! - globalEventHandler");
        }
      }
    };

    globalEventHandler.SetValue(null, value);
  }
}

hierarchyWindowItemOnGUI.cs

[InitializeOnLoad]
public static class StaticEvents {
  static StaticEvents() {
    EditorApplication.hierarchyWindowItemOnGUI += (int instanceID, Rect selectionRect) => {
      // Triggered only when a hierarchy item is selected + the hierarchy is focused

      if (instanceID == Selection.activeInstanceID) {
        var e = Event.current;
        if (e.type == EventType.KeyDown) {
          if (e.keyCode == KeyCode.LeftShift) {
            Debug.Log("SHIFT DOWN EVERYBODY!! - hierarchyWindowItemOnGUI");
          }
        }
      }
    };
  }
}

modifierKeysChanged.cs

[InitializeOnLoad]
public static class StaticEvents {
  static StaticEvents() {
    EditorApplication.modifierKeysChanged += () => {
      // No events, only "something" changed
    };
  }
}

onSceneGUIDelegate.variantA.cs

[InitializeOnLoad]
public static class StaticEvents {
  static StaticEvents() {
    SceneView.onSceneGUIDelegate += view => {
      // Mentioned in one thread: Get a control ID to not mess with other controls
      // Does not seem to actually work in Unity 2018.1 on Linux - but theoretically should work
      int controlID = GUIUtility.GetControlID(FocusType.Keyboard);
      if (Event.current.GetTypeForControl(controlID) == EventType.KeyDown) {
        if (Event.current.shift) {
          Debug.Log("SHIFT DOWN EVERYBODY!! - onSceneGUIDelegate with Control ID A");
          // Causes repaint & accepts event has been handled
          Event.current.Use();
        }
        if (Event.current.keyCode == KeyCode.LeftShift) {
          Debug.Log("SHIFT DOWN EVERYBODY!! - onSceneGUIDelegate with Control ID B");
          // Causes repaint & accepts event has been handled
          Event.current.Use();
        }
      }
    };
  }
}

onSceneGUIDelegate.variantB.cs

[InitializeOnLoad]
public static class StaticEvents {
  static StaticEvents() {
    SceneView.onSceneGUIDelegate += view => {
      // Same thing as variantA, but just directly process the event and not use it (watch only)
      // Does not seem to actually work in Unity 2018.1 on Linux - but theoretically should work
      var e = Event.current;

      if (e == null) {
        return;
      }

      if (e.type == EventType.KeyDown) {
        if (e.keyCode == KeyCode.LeftShift) {
          Debug.Log("SHIFT DOWN EVERYBODY!! - onSceneGUIDelegate");
        }
      }

      if (e.modifiers.HasFlag(EventModifiers.Shift)) {
        if (e.keyCode == KeyCode.LeftShift) {
          Debug.Log("SHIFT DOWN EVERYBODY!! - onSceneGUIDelegate as Modifier");
        }
      }
    };
  }
}