Error in displaying the ribbon in Microsoft Word when using the preview via PreviewHandler

Question

We display Microsoft Word documents (docx) in a viewer. The PreviewHandler is used for this purpose. In Microsoft Word 2021 the following problem occurs: the ribbon in Microsoft Word does not contain any content when Microsoft Word is started with a template after displaying it in our viewer.

Microsoft Word without content in the ribbon

Reproduction: All instances of Microsoft Word must be closed. Then a Microsoft Word file is displayed in the viewer. Afterwards Microsoft Word has to be started. Here the ribbon is still displayed correctly. From the started instance, a new Microsoft Word document must then be created from a template, for example "Take a Tour". This instance then shows the error behavior.

However, the error cannot be reproduced with all Microsoft Word versions. The error does not occur under the following costellations: Windows 10 with Word 365 MSO (version 2306 build 16.0.16529.20226) 64 bit and Windows 2012 R2 with Word 2016 MSO (version 2302 build 16.0.16130.20332) 64 bit. Reproducible with Windows 11 with Word 2021 MSO (version 2307 build 16.0.16626.20170) 64 bit

We use the following code to display a Microsoft Word document using the PreviewHandler:

using Microsoft.Win32;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

namespace WordPreviewTest
{
    public partial class Form1 : Form
    {
        private IPreviewHandler? previewHandler = null;
        private COMStream stream;
        private bool isInitialized = false;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            const string fileExtension = ".docx";
            const string BaseRegistryKey = "SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\PreviewHandlers";
            const string BaseClsIDKey = @"HKEY_CLASSES_ROOT\{0}\shellex\{{8895b1c6-b41f-4c1c-a562-0d564250836f}}";

            bool foundPreviewHandler = false;
            string? extId = Registry.GetValue(string.Format(BaseClsIDKey, fileExtension), null, null) as string;
            if (extId != null)
            {
                using RegistryKey? handlersKey = Registry.LocalMachine.OpenSubKey(BaseRegistryKey);
                if (handlersKey != null)
                {
                    string[] handlerKeyValues = handlersKey.GetValueNames();
                    foreach (string id in handlerKeyValues)
                    {
                        if (extId == id)
                        {
                            foundPreviewHandler = true;
                        }
                    }
                }

                if (foundPreviewHandler)
                {
                    Guid guid = new Guid(extId);
                    Type? previewerType = Type.GetTypeFromCLSID(guid);
                    if (previewerType != null)
                    {
                        object? comInstance = Activator.CreateInstance(previewerType);
                        if (comInstance != null)
                        {
                            previewHandler = comInstance as IPreviewHandler;
                            IInitializeWithFile? initializeWithFile = comInstance as IInitializeWithFile;
                            IInitializeWithStream? initializeWithStream = comInstance as IInitializeWithStream;
                            if (initializeWithFile != null)
                            {
                                initializeWithFile.Initialize(@"C:\Temp\Word2016.docx", 0);
                                isInitialized = true;
                            }
                            else if (initializeWithStream != null)
                            {
                                stream = new COMStream(File.Open(@"C:\Temp\Word2016.docx", FileMode.Open, FileAccess.Read));
                                initializeWithStream.Initialize((IStream)stream, 0);
                                isInitialized = true;
                            }
                        }
                    }
                }
            }
        }

        private void Form1_Shown(object sender, EventArgs e)
        {
            if (previewHandler != null && isInitialized)
            {
                RECT rect = new RECT()
                {
                    left = 0,
                    top = 0,
                    bottom = panel1.Height,
                    right = panel1.Width
                };
                previewHandler.SetWindow(panel1.Handle, ref rect);
                previewHandler.DoPreview();
            }
        }

        private void Form1_Resize(object sender, EventArgs e)
        {
            if (previewHandler != null && isInitialized)
            {
                RECT rect = new RECT()
                {
                    left = 0,
                    top = 0,
                    bottom = panel1.Height,
                    right = panel1.Width
                };

                previewHandler.SetRect(ref rect);
            }
        }

        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            previewHandler?.Unload();
        }
    }
}

I think that the problem is due to the use of the PreviewHandler, because Windows Explorer with its preview does not trigger this problem.

The thinking went into releasing the resources that the PreviewHandler takes up resources from Microsoft Word and does not release them. Therefore the use of the PreviewHandler was examined, but no solution could be found for this.