Prevent Focus Loss in Unity OnSceneGUI

SomeComponentEditor.cs

/**
 * This is an Editor class for some Component class
 */
[CustomEditor(typeof(SomeComponent))]
public class SomeComponentEditor : Editor {

  // OnSceneGUI() is often used to paint or interact with objects in the Scene
  // at edit time. Here we show how to prevent clicking on a game scene from
  // resulting in a change of focused/selected element in the Scene hierarchy.
  private void OnSceneGUI() {
    if (Event.current.type == EventType.MouseDown) {
      // get the controlID of the passively focused item (typically the selected Gameobject)
      int controlId = GUIUtility.GetControlID(FocusType.Passive);

      // YOUR LOGIC HERE:
      // perform you processing here...

      // force focus (hot control) back to the original object, even if the click event would shift it
      GUIUtility.hotControl = controlId;  // force focus on the current control
      Event.current.Use();  // consume the event so it doesn't get passed on to other handlers
    }    
  }  
}

This is a technique that can be used when writing Unity component editor classes.
Editors are used to control how the behavior of game objects (or their components) works in the Editor window.

Occasionally, we want to create editors that response to things like mouse events (up, down, move etc) but swallow these events after processing them. Essentially, we don't want the selected object in the scene view to change while (or after) we do things like paint on terrain, place objects in the scene, etc.

The GUIUtility class provides methods like GetControlID() and .hotControl which are normally used when creating your own UI controls. However, we can use them in such cases to control which scene elements retain focus after mouse interaction.