如何防止WinForm弹出窗口阴影的Z-order层级异常？

问题现象 #

使用ShowDialog()显示自定义模态弹窗（通过CS_DROPSHADOW类样式添加阴影）时：

• 初始状态阴影显示正常：
  
• 点击模态窗口外的主窗口区域后，阴影被主窗口遮挡：
  
  切换到其他Windows进程再切回程序后，阴影恢复正常。

原代码 #

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TestShadow
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            PopupForm popupForm = new PopupForm();
            popupForm.ShowInTaskbar = false;
            popupForm.StartPosition = FormStartPosition.Manual;
            popupForm.FormBorderStyle = FormBorderStyle.None;
            StartPosition = FormStartPosition.Manual;
            var topLeft = button1.PointToScreen(new Point(0, 0));
            popupForm.Location = new Point(topLeft.X, topLeft.Y + button1.Height);
            popupForm.ShowDialog();
        }
    }
    public class PopupForm : Form
    {
        const int WS_EX_TOOLWINDOW = 0x00000080;
        protected override CreateParams CreateParams
        {
            get
            {
                const int CS_NOCLOSE = 0x200;
                const int CS_DROPSHADOW = 0x20000;
                CreateParams cp = base.CreateParams;
                cp.ClassStyle |= CS_NOCLOSE;
                cp.ClassStyle |= CS_DROPSHADOW;
                return cp;
            }
        }
    }
}

解决方案 #

方法1：调整弹窗扩展样式并处理主窗口激活事件

1. 给弹窗添加WS_EX_NOACTIVATE和WS_EX_TOOLWINDOW扩展样式，避免弹窗在主窗口被点击时失去层级优势；
2. 在主窗口的Activated事件中强制弹窗重绘，确保阴影始终显示。

修改后的代码：

using System;
using System.Windows.Forms;

namespace TestShadow
{
    public partial class Form1 : Form
    {
        private PopupForm _popupForm;
        
        public Form1()
        {
            InitializeComponent();
            this.Activated += Form1_Activated;
        }
        
        private void Form1_Activated(object sender, EventArgs e)
        {
            // 主窗口激活时强制弹窗重绘，恢复阴影显示
            _popupForm?.Invalidate();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            _popupForm = new PopupForm();
            _popupForm.ShowInTaskbar = false;
            _popupForm.StartPosition = FormStartPosition.Manual;
            _popupForm.FormBorderStyle = FormBorderStyle.None;
            var topLeft = button1.PointToScreen(new Point(0, 0));
            _popupForm.Location = new Point(topLeft.X, topLeft.Y + button1.Height);
            _popupForm.ShowDialog();
            _popupForm = null;
        }
    }

    public class PopupForm : Form
    {
        const int WS_EX_TOOLWINDOW = 0x00000080;
        const int WS_EX_NOACTIVATE = 0x08000000;

        protected override CreateParams CreateParams
        {
            get
            {
                const int CS_NOCLOSE = 0x200;
                const int CS_DROPSHADOW = 0x20000;
                CreateParams cp = base.CreateParams;
                cp.ClassStyle |= CS_NOCLOSE | CS_DROPSHADOW;
                // 添加扩展样式，避免弹窗被主窗口遮挡
                cp.ExStyle |= WS_EX_TOOLWINDOW | WS_EX_NOACTIVATE;
                return cp;
            }
        }
    }
}

方法2：使用Windows API强制弹窗保持顶层

如果方法1仍有问题，可以通过SetWindowPosAPI强制弹窗始终在主窗口上方：
在PopupForm类中添加以下代码：

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

private const uint SWP_NOMOVE = 0x0002;
private const uint SWP_NOSIZE = 0x0001;
private const IntPtr HWND_TOPMOST = new IntPtr(-1);

protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    // 强制弹窗保持顶层
    SetWindowPos(this.Handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}

原因说明 #

• CS_DROPSHADOW的阴影由系统绘制在窗口区域外，当主窗口试图获取焦点（即使被模态阻止）时，系统可能临时调整窗口Z序或重绘层级，导致主窗口覆盖弹窗阴影区域；
• WS_EX_NOACTIVATE确保弹窗不会被激活，避免层级变化；WS_EX_TOOLWINDOW让弹窗更符合工具窗口特性，减少系统对其层级的干预；
• 主窗口激活时强制弹窗重绘，能触发系统重新绘制阴影，恢复正常显示。

内容的提问来源于stack exchange，提问作者Michael