Canvas+Mask嵌套
如果子级Canvas上面使用了Override Sorting，会导致父级的Mask，无法影响子级

原因：
Mask在子级Canvas上，进行了模板重置，所以增加一个Mask子类MaskAAA：

using System;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.Rendering;
using UnityEngine.Serialization;

namespace UnityEngine.UI
{
    [AddComponentMenu("UI/MaskAAA", 13)]
    [ExecuteAlways]
    [RequireComponent(typeof(RectTransform))]
    [DisallowMultipleComponent]
    /// <summary>
    /// A component for masking children elements.
    /// </summary>
    /// <remarks>
    /// By using this element any children elements that have masking enabled will mask where a sibling Graphic would write 0 to the stencil buffer.
    /// </remarks>
    public class MaskAAA : UnityEngine.UI.Mask
    {
        public bool m_ShowMaskGraphic { get { return showMaskGraphic; } }
        private Material m_MaskMaterial;
        private Material m_UnmaskMaterial;

        /// Stencil calculation time!
        public override Material GetModifiedMaterial(Material baseMaterial)
        {
            if (!MaskEnabled())
                return baseMaterial;

            var rootSortCanvas = MaskUtilities.FindRootSortOverrideCanvas(transform);
            var stencilDepth = MaskUtilities.GetStencilDepth(transform, rootSortCanvas);
            if (stencilDepth >= 8)
            {
                Debug.LogWarning("Attempting to use a stencil mask with depth > 8", gameObject);
                return baseMaterial;
            }

            int desiredStencilBit = 1 << stencilDepth;

            // if we are at the first level...
            // we want to destroy what is there
            if (desiredStencilBit == 1)
            {
                var maskMaterial = StencilMaterial.Add(baseMaterial, 1, StencilOp.Replace, CompareFunction.Always, m_ShowMaskGraphic ? ColorWriteMask.All : 0);
                StencilMaterial.Remove(m_MaskMaterial);
                m_MaskMaterial = maskMaterial;

                // var unmaskMaterial = StencilMaterial.Add(baseMaterial, 1, StencilOp.Zero, CompareFunction.Always, 0);
                // StencilMaterial.Remove(m_UnmaskMaterial);
                // m_UnmaskMaterial = unmaskMaterial;
                // graphic.canvasRenderer.popMaterialCount = 1;
                // graphic.canvasRenderer.SetPopMaterial(m_UnmaskMaterial, 0);

                return m_MaskMaterial;
            }

            //otherwise we need to be a bit smarter and set some read / write masks
            var maskMaterial2 = StencilMaterial.Add(baseMaterial, desiredStencilBit | (desiredStencilBit - 1), StencilOp.Replace, CompareFunction.Equal, m_ShowMaskGraphic ? ColorWriteMask.All : 0, desiredStencilBit - 1, desiredStencilBit | (desiredStencilBit - 1));
            StencilMaterial.Remove(m_MaskMaterial);
            m_MaskMaterial = maskMaterial2;

            graphic.canvasRenderer.hasPopInstruction = true;
            var unmaskMaterial2 = StencilMaterial.Add(baseMaterial, desiredStencilBit - 1, StencilOp.Replace, CompareFunction.Equal, 0, desiredStencilBit - 1, desiredStencilBit | (desiredStencilBit - 1));
            StencilMaterial.Remove(m_UnmaskMaterial);
            m_UnmaskMaterial = unmaskMaterial2;
            graphic.canvasRenderer.popMaterialCount = 1;
            graphic.canvasRenderer.SetPopMaterial(m_UnmaskMaterial, 0);

            return m_MaskMaterial;
        }
    }
}

然后增加一个新的UI材质

把Stencil Comparison设置为6，放到子Canvas的Image上，即可