Make blackboard only be draggable from the header to avoid conflicts with fields

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Blackboard/BlackboardProvider.cs

                 moveItemRequested = MoveItemRequested
             };
 
-            m_WindowDraggable = new WindowDraggable();
+            m_WindowDraggable = new WindowDraggable(blackboard.shadow.Children().First().Q("header"));
             blackboard.AddManipulator(m_WindowDraggable);
 
             m_ResizeBorderFrame = new ResizeBorderFrame(blackboard) { name = "resizeBorderFrame" };

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Manipulators/WindowDraggable.cs

-using System;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
         Vector2 m_LocalMosueOffset;
         Rect m_PreviousParentRect;
 
+        VisualElement m_Handle;
+
-        public WindowDraggable(bool resizeWithParentwindow = false)
+        public WindowDraggable(VisualElement handle = null, bool resizeWithParentwindow = false)
+            m_Handle = handle;
             m_ResizeWithParentWindow = resizeWithParentwindow;
             m_Active = false;
             m_PreviousParentRect = new Rect(0f, 0f, 0f, 0f);
         {
-            target.RegisterCallback(new EventCallback<MouseDownEvent>(OnMouseDown), Capture.NoCapture);
-            target.RegisterCallback(new EventCallback<MouseMoveEvent>(OnMouseMove), Capture.NoCapture);
-            target.RegisterCallback(new EventCallback<MouseUpEvent>(OnMouseUp), Capture.NoCapture);
+            if (m_Handle == null)
+                m_Handle = target;
+            m_Handle.RegisterCallback(new EventCallback<MouseDownEvent>(OnMouseDown), Capture.NoCapture);
+            m_Handle.RegisterCallback(new EventCallback<MouseMoveEvent>(OnMouseMove), Capture.NoCapture);
+            m_Handle.RegisterCallback(new EventCallback<MouseUpEvent>(OnMouseUp), Capture.NoCapture);
-            target.UnregisterCallback(new EventCallback<MouseDownEvent>(OnMouseDown), Capture.NoCapture);
-            target.UnregisterCallback(new EventCallback<MouseMoveEvent>(OnMouseMove), Capture.NoCapture);
-            target.UnregisterCallback(new EventCallback<MouseUpEvent>(OnMouseUp), Capture.NoCapture);
+            m_Handle.UnregisterCallback(new EventCallback<MouseDownEvent>(OnMouseDown), Capture.NoCapture);
+            m_Handle.UnregisterCallback(new EventCallback<MouseMoveEvent>(OnMouseMove), Capture.NoCapture);
+            m_Handle.UnregisterCallback(new EventCallback<MouseUpEvent>(OnMouseUp), Capture.NoCapture);
-            m_LocalMosueOffset = target.WorldToLocal(evt.mousePosition);
+            m_LocalMosueOffset = m_Handle.WorldToLocal(evt.mousePosition);
-            target.TakeMouseCapture();
-            evt.StopPropagation();
+            m_Handle.TakeMouseCapture();
+            evt.StopImmediatePropagation();
         }
 
         void OnMouseMove(MouseMoveEvent evt)
 
             m_Active = false;
 
-            if (target.HasMouseCapture())
+            if (m_Handle.HasMouseCapture())
-                target.ReleaseMouseCapture();
+                m_Handle.ReleaseMouseCapture();
-            evt.StopPropagation();
+            evt.StopImmediatePropagation();
 
             RefreshDocking();