source: Common/TabControlFocus.cs

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

namespace Common
{
    class FocusState
    {
        public Control control;
        public int scrollPosVertical;
        public int scrollPosHorizontal;
    }

    class TabControlFocus
    {
        private List<FocusState> lastFocusedControls = new List<FocusState>();
        private int focusedTabIndex = 0;
        public TabControl tabControl = null;

        private void UpdateListSize()
        {
            if (tabControl != null)
            {
                while (lastFocusedControls.Count < tabControl.TabPages.Count)
                    lastFocusedControls.Add(new FocusState());
                while (lastFocusedControls.Count > tabControl.TabPages.Count)
                    lastFocusedControls.RemoveAt(lastFocusedControls.Count - 1);
            }
        }

        public void SetupTabControl(TabControl tabControl)
        {
            this.tabControl = tabControl;
            tabControl.SelectedIndexChanged += delegate (object sender, EventArgs e)
            {
                TabControl parentTabControl = (sender as TabControl);
                Control control = null;
                UpdateListSize();
                if (parentTabControl.SelectedIndex != -1)
                {
                    control = lastFocusedControls[parentTabControl.SelectedIndex].control;
                    if (control != null)
                    {
                        control.Focus();

                        // Restore strollbars position of nearest parent panel with autoscroll
                        while (control.Parent != null)
                        {
                            if ((control.Parent is Panel) && (control.Parent as Panel).AutoScroll)
                            {
                                (control.Parent as Panel).VerticalScroll.Value = lastFocusedControls[parentTabControl.SelectedIndex].scrollPosVertical;
                                (control.Parent as Panel).HorizontalScroll.Value = lastFocusedControls[parentTabControl.SelectedIndex].scrollPosHorizontal;
                                (control.Parent as Panel).PerformLayout();
                                break;
                            }
                            control = control.Parent;
                        }
                    }
                }
                focusedTabIndex = parentTabControl.SelectedIndex;
            };
        }

        private void TrackLostFocus(object sender, EventArgs e)
        {
            Control item = (Control)sender;
            Control control = (Control)sender;

            UpdateListSize();
            if (!(control is Panel))
            while (item.Parent != null)
            {
                if (item.Parent is TabPage)
                {
                    int index = tabControl.TabPages.IndexOf(item.Parent as TabPage);
                    if (index != -1)
                    {
                        if (focusedTabIndex == tabControl.SelectedIndex)
                        {
                            lastFocusedControls[index].control = control;
                            
                            // Store scrollbars position of nearest parent panel with autoscroll
                            while (control.Parent != null)
                            {
                                if ((control.Parent is Panel) && (control.Parent as Panel).AutoScroll)
                                {
                                    lastFocusedControls[index].scrollPosVertical = (control.Parent as Panel).VerticalScroll.Value;
                                    lastFocusedControls[index].scrollPosHorizontal = (control.Parent as Panel).HorizontalScroll.Value;
                                    break;
                                }
                                control = control.Parent;
                            }
                            break;
                        }
                    }
                }
                item = item.Parent;
            }
        }

        public void TrackLostFocusOnChildControls(Control.ControlCollection controls)
        {
            foreach (Control control in controls)
            {
                control.LostFocus += new EventHandler(TrackLostFocus);

                Control.ControlCollection childControls = control.Controls;
                if (childControls != null)
                    TrackLostFocusOnChildControls(childControls);
            }
        }
    }
}