Add callback in resize handles that notifies when the mouse button is released

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

-using System.Collections;
+using System;
+using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Experimental.UIElements;
 public class ResizeBorderFrame : VisualElement
 {
+    public Action OnResizeFinished;
+
     public ResizeBorderFrame(VisualElement target)
     {
         pickingMode = PickingMode.Ignore;
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.TopLeft));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.Top));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.TopRight));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.Right));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.BottomRight));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.Bottom));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.BottomLeft));
-        Add(new ResizeSideHandle(target, ResizeHandleAnchor.Left));
+        ResizeSideHandle topLeft =  new ResizeSideHandle(target, ResizeHandleAnchor.TopLeft);
+        ResizeSideHandle top =  new ResizeSideHandle(target, ResizeHandleAnchor.Top);
+        ResizeSideHandle topRight =  new ResizeSideHandle(target, ResizeHandleAnchor.TopRight);
+        ResizeSideHandle right = new ResizeSideHandle(target, ResizeHandleAnchor.Right);
+        ResizeSideHandle bottomRight = new ResizeSideHandle(target, ResizeHandleAnchor.BottomRight);
+        ResizeSideHandle bottom = new ResizeSideHandle(target, ResizeHandleAnchor.Bottom);
+        ResizeSideHandle bottomLeft = new ResizeSideHandle(target, ResizeHandleAnchor.BottomLeft);
+        ResizeSideHandle left = new ResizeSideHandle(target, ResizeHandleAnchor.Left);
+
+        topLeft.OnResizeFinished += HandleResizefinished;
+        top.OnResizeFinished += HandleResizefinished;
+        topRight.OnResizeFinished += HandleResizefinished;
+        right.OnResizeFinished += HandleResizefinished;
+        bottomRight.OnResizeFinished += HandleResizefinished;
+        bottom.OnResizeFinished += HandleResizefinished;
+        bottomLeft.OnResizeFinished += HandleResizefinished;
+        left.OnResizeFinished += HandleResizefinished;
+
+        Add(topLeft);
+        Add(top);
+        Add(topRight);
+        Add(right);
+        Add(bottomRight);
+        Add(bottom);
+        Add(bottomLeft);
+        Add(left);
+    }
+
+    void HandleResizefinished()
+    {
+        if (OnResizeFinished != null)
+        {
+            OnResizeFinished();
+        }
     }
 }

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

-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Experimental.UIElements;
 
     {
         VisualElement m_ResizeTarget;
 
+        public Action OnResizeFinished;
+
         public ResizeSideHandle(VisualElement resizeTarget, ResizeHandleAnchor anchor)
         {
             m_ResizeTarget = resizeTarget;
             bool moveWhileResizeVertical = anchor == ResizeHandleAnchor.TopLeft || anchor == ResizeHandleAnchor.TopRight || anchor == ResizeHandleAnchor.Top;
 
             this.AddManipulator(new Draggable(mouseDelta => OnResize(mouseDelta, resizeDirection, moveWhileResizeHorizontal, moveWhileResizeVertical)));
+            RegisterCallback<MouseUpEvent>(HandleDraggableMouseUp);
         }
 
         void OnResize(Vector2 resizeDelta, ResizeDirection direction, bool moveWhileResizeHorizontal, bool moveWhileresizerVertical)
             newLayout.y = Mathf.Min(newLayout.y + normalizedResizeDelta.y, previousFarY - minSize.y);
 
             m_ResizeTarget.layout = newLayout;
+        }
+
+        void HandleDraggableMouseUp(MouseUpEvent evt)
+        {
+            if (OnResizeFinished != null)
+            {
+                OnResizeFinished();
+            }
         }
     }
 }