I reverted the change that caused the OnSceneGUI event to exit early after a selection change, restoring the behaviour of previous Unity versions.

However, I would not recommend this method of preventing selection changes in the scene view. A more efficient way would be to register a default control to prevent picking from allowing any selection change in the first place.

Example:

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(templateScript))]
public class ERTemplateEditor : Editor
{
void OnSceneGUI()
{
InterceptPicking();
}

// Example code showing how to prevent selection changes from clicking in the Scene View with an object selected.
static void InterceptPicking()
{
var evt = Event.current;
var id = GUIUtility.GetControlID(FocusType.Passive);

switch (evt.type)
{
case EventType.Layout:
// If the event is left mouse button and no navigation key modifiers are pressed, add a default control.
// A default control will let any handles or ui in the scene take priority, but if nothing is closer
// it will be chosen as the active control. Since picking is also a default control but tested after
// all user code, this gives us the chance to intercept the event.
if(evt.button == 0 && !evt.alt)
HandleUtility.AddDefaultControl(id);
break;

case EventType.MouseDown:
if (HandleUtility.nearestControl == id)
{
evt.Use();
GUIUtility.hotControl = id;
}
break;

case EventType.MouseUp:
if (GUIUtility.hotControl == id)
{
evt.Use();
GUIUtility.hotControl = 0;
}
break;
}
}
}