evershade/Assets/DraggableUIPanel.cs

using UnityEngine;
using UnityEngine.EventSystems;

public class DraggablePanel : MonoBehaviour, IDragHandler, IBeginDragHandler, IEndDragHandler
{
    public GameObject dragHandle;
    public RectTransform rectTransform;
    public Canvas canvas;
    public Vector2 offset;
    public bool isDragging;

    private void Awake()
    {
        // rectTransform = GetComponent<RectTransform>();
        canvas = GetComponentInParent<Canvas>();
    }

    public void OnBeginDrag(PointerEventData eventData)
    {
        if (IsPointerOverDragHandle(eventData))
        {
            isDragging = true;
            RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform,
                eventData.position, eventData.pressEventCamera, out var localCursorPoint);
            offset = rectTransform.anchoredPosition - localCursorPoint;
        }
    }

    public void OnDrag(PointerEventData eventData)
    {
        if (isDragging)
            if (RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform,
                    eventData.position, eventData.pressEventCamera, out var localCursorPoint))
                rectTransform.anchoredPosition = localCursorPoint + offset;
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        isDragging = false;
    }

    private bool IsPointerOverDragHandle(PointerEventData eventData)
    {
        return dragHandle != null && RectTransformUtility.RectangleContainsScreenPoint(
            dragHandle.GetComponent<RectTransform>(),
            eventData.position,
            eventData.pressEventCamera);
    }
}