Draggable regions doesn't work with VSTO

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Description

I am using WebView2 with WinForms to make VSTO PowerPoint Addin.
I implemented draggable regions (Issue #200),
and it works in the WinForms project,
but it doesn't work with PowerPoint.

I found SendMessage takes a long time so that users cannot drag the window.

https://user-images.githubusercontent.com/16956287/113473541-15220d80-94a5-11eb-9f7b-84756e0f78a3.mov

Version
SDK: 1.0.774.44
Runtime: 89.0.774.63
Framework: WinForms
OS: Win 10 Home (1909)

Reproducing Codes

using System;
using System.Windows.Forms;
using System.IO;

using Microsoft.Web.WebView2.WinForms;
using Microsoft.Web.WebView2.Core;
using System.Runtime.InteropServices;

namespace WebViewMouseEventTest
{
    public partial class ThisAddIn
    {
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            new Form1().Show();
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
    }

    public class Form1 : Form
    {
        public const int WM_NCLBUTTONDOWN = 0xA1;
        public const int HT_CAPTION = 0x2;

        [DllImportAttribute("user32.dll")]
        public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);
        [DllImportAttribute("user32.dll")]
        public static extern int PostMessageA(IntPtr hWnd, int Msg, int wParam, int lParam);
        [DllImportAttribute("user32.dll")]
        public static extern bool ReleaseCapture();

        public delegate void F();
        private WebView2 webview2;
        public Form1()
        {
            InitializeComponent();
        }

        private void InitializeComponent()
        {
            this.MouseDown += Form1_MouseDown;
            webview2 = new WebView2();
            Controls.Add(webview2);
            webview2.Top = 50;
            webview2.Width = 100;
            webview2.Height = 100;
            var env = CoreWebView2Environment.CreateAsync(null, Path.Combine(Path.GetTempPath(), "webview2")).Result;
            webview2.EnsureCoreWebView2Async(env)
                .ContinueWith((t) => this.Invoke(new F(_InitializeAsync)));
            webview2.Source = new Uri(Path.GetFullPath(@"index.html"));
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            ReleaseCapture();
            SendMessage(Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
        }


        private void _InitializeAsync()
        {
            this.webview2.CoreWebView2.WebMessageReceived += CoreWebView2_WebMessageReceived;
        }

        private void CoreWebView2_WebMessageReceived(object sender, CoreWebView2WebMessageReceivedEventArgs e)
        {
            ReleaseCapture();
            SendMessage(Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
        }
    }
}

<!DOCTYPE html>
<html style="height: 100%; margin: 0; padding: 0;" lang="en" xmlns="http://www.w3.org/1999/xhtml">
<body style="background-color: yellow; height: 100%; margin: 0; padding: 0;">
    WebView
    <script>
    window.addEventListener('DOMContentLoaded', () => {
        console.log('DOMContentLoaded');
        document.body.addEventListener('mousedown', evt => {
            console.log('mousedown');
            chrome.webview.postMessage('mousedown');
            evt.preventDefault();
            evt.stopPropagation();
        });
    })
    </script>
</body>
</html>

AB#32377080

Start with the provided WinForms reproduction, especially CoreWebView2_WebMessageReceived, which calls ReleaseCapture and SendMessage after a WebView2 message. Compare the behavior in a regular WinForms project with the reported VSTO PowerPoint add-in case. Done means identifying why the draggable region fails in that environment and establishing whether it can work reliably; the issue names no test or project file.

Written by the indexing model from the issue text.

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