CellOverlayWindow.xaml



<Window x:Class="YourAddIn.CellOverlayWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        WindowStyle="None" ShowInTaskbar="False" Topmost="True"

        ResizeMode="NoResize"

        Focusable="True"

        AllowsTransparency="False"                  <!-- keep focusable -->

        Background="White"                          <!-- or your theme -->

        Width="100" Height="24">

  <Grid Margin="0">

    <TextBox x:Name="Editor"

             VerticalAlignment="Center"

             BorderThickness="0"

             Padding="2"

             Focusable="True"

             MinWidth="40"

             FontSize="12"

             KeyDown="EditorKeyDown"

             TextChanged="EditorTextChanged"/>

  </Grid>



CellOverlayWindow.xaml.cs



using System;

using System.Runtime.InteropServices;

using System.Windows;

using System.Windows.Input;

using System.Windows.Interop;

namespace YourAddIn

{

    public partial class CellOverlayWindow : Window

    {

        public CellOverlayWindow()

        {

            InitializeComponent();

            WindowStyle = WindowStyle.None;

            ShowInTaskbar = false;

            Topmost = true;

            Focusable = true;

        }

        public void FocusEditorDeferred()

        {

            // Defer to run AFTER Excel’s own activation completes

            Dispatcher.BeginInvoke(new Action(() =>

            {

                Activate();

                Editor.Focus();

                Keyboard.Focus(Editor);

                Editor.SelectAll();

            }), System.Windows.Threading.DispatcherPriority.ApplicationIdle);

        }

        public void ForceWin32Focus()

        {

            var hwnd = new WindowInteropHelper(this).Handle;

            SetForegroundWindow(hwnd);

            SetFocus(hwnd);

        }

        [DllImport("user32.dll")] static extern IntPtr SetFocus(IntPtr hWnd);

        [DllImport("user32.dll")] static extern bool SetForegroundWindow(IntPtr hWnd);

        // Optional: handle Enter/Esc here; wire to YourAddIn via events if preferred

        private void EditorKeyDown(object sender, KeyEventArgs e)

        {

            // Prevent F2 (Excel in-cell edit) while overlay is active

            if (e.Key == Key.F2)

            {

                e.Handled = true;

                return;

            }

            if (e.Key == Key.Enter)

            {

                e.Handled = true;

                CommitAndHide?.Invoke(this, EventArgs.Empty);

            }

            else if (e.Key == Key.Escape)

            {

                e.Handled = true;

                               CancelAndHide?.Invoke(this, EventArgs.Empty);

            }

        }

        public event EventHandler CommitAndHide;

        public event EventHandler CancelAndHide;

        private void EditorTextChanged(object sender, System.Windows.Controls.TextChangedEventArgs e)

        {

            // Your inline validation or live preview hooks (optional)

        }

    }