Simple Popup Control

            ToolStripDropDown tspopUp = new ToolStripDropDown();
            tspopUp.Margin = Padding.Empty;
            tspopUp.Padding = Padding.Empty;
            ToolStripControlHost host = new ToolStripControlHost(CategoryPanel);
            host.Margin = Padding.Empty;
            host.Padding = Padding.Empty;
            tspopUp.Items.Add(host);
            tspopUp.Show(GroupBox1,txtcategory.Location);

        Popup toolTip;
        CustomToolTip customToolTip;
        Timer toolTipTimer = new Timer();
        TreeNode toolTipNode = null;

            toolTip = new Popup(customToolTip = new CustomToolTip());
            toolTip.AutoClose = false;
            toolTip.FocusOnOpen = false;
            toolTip.DropShadowEnabled = true;
            toolTip.ShowingAnimation = toolTip.HidingAnimation = PopupAnimations.Blend;
            toolTip.AutoSize = true;
            toolTip.Resizable = true;

            toolTipTimer.Interval = 1500;
            toolTipTimer.Tick += new EventHandler(toolTipTimer_Tick);

            // Do not show the built in tooltips for the tree nodes
            tv.ShowNodeToolTips = false;

        int nStartX = 0;    // Starting X position of the custom tooltip
        int nStartY = 0;    // Starting Y position of the custom tooltip
        int nLastX = 0;     // Last known mouse X position
        int nLastY = 0;     // Last known mouse Y position

        void toolTipTimer_Tick(object sender, EventArgs e)
        {
            if (toolTipNode != null)
            {
                toolTipTimer.Stop();
                customToolTip.TipText = toolTipNode.Text.Trim();
                toolTip.Show(this, nLastX + 10, nLastY + 50);
            }
        }

        private void tv_MouseMove(object sender, MouseEventArgs e)
        {
            TreeNode node = tv.GetNodeAt(e.X, e.Y);

            if (node == null)
            {
                toolTipTimer.Stop();
                if (toolTip.Visible)
                {
                    toolTip.Close();
                }
            }
            else
            {
                if (!toolTipTimer.Enabled)
                {
                    toolTipNode = node;
                    toolTipTimer.Start();
                    nStartX = nLastX;
                    nStartY = nLastY;
                }
                else if ((e.X < nStartX-5) || (e.X > nStartX+5) || (e.Y < nStartY-3) || (e.Y > nStartY+3))
                {
                    toolTipTimer.Stop();
                    if (toolTip.Visible)
                        toolTip.Close();
                }
            }

            nLastX = e.X;
            nLastY = e.Y;

        }

        private void tv_MouseLeave(object sender, System.EventArgs e)
        {
            toolTipTimer.Stop();
            toolTip.Close();
        }

public partial class Form1 : Form
{
    public Form1()
    {
        Control.CheckForIllegalCrossThreadCalls = true;
        InitializeComponent();
        _popup = new Popup(_popupPanel = new Panel()
            {
                AutoSize = true,
                AutoSizeMode = AutoSizeMode.GrowAndShrink,
                BackColor = SystemColors.WindowFrame,
                Padding = new Padding(0),
            })
        {
            AutoSize = true,
            FocusOnOpen = false,
        };
        _popupLabel = new Label()
        {
            AutoSize = true,
            BackColor = SystemColors.Info,
            BorderStyle = BorderStyle.FixedSingle,
            Dock = DockStyle.Fill,
            ForeColor = SystemColors.InfoText,
            TextAlign = ContentAlignment.MiddleCenter,
        };
        _popupPanel.Controls.Add(_popupLabel);
        _popup.Opened += (s, e) =>
        {
            object o = new object();
            System.Threading.Timer timer = null;
            Action close = () =>
            {
                if (_popupLabel.RectangleToScreen(_popupLabel.ClientRectangle).Contains(Cursor.Position)) return;
                if (timer != null)
                {
                    lock (o)
                    {
                        if (timer != null)
                        {
                            timer.Dispose();
                            timer = null;
                            _popup.Close();
                        }
                    }
                }
            };
            _popupLabel.MouseLeave += (s2, e2) => close();
            timer = new System.Threading.Timer((_) => Invoke(close), null, 2500, -1);
        };
    }

    Popup _popup;
    Panel _popupPanel;
    Label _popupLabel;

    private void treeView1_NodeMouseHover(object sender, TreeNodeMouseHoverEventArgs e)
    {
        _popupLabel.Text = e.Node.ToolTipText;
        _popup.Size = _popupLabel.Size;
        _popup.Show(treeView1.RectangleToScreen(e.Node.Bounds).Location);
        // or
        //_popup.Show(treeView1, e.Node.Bounds);
    }
}

Galatei wrote:

2. Popup windows are different from POP-UPs. POP-UPs are exploited by advertising companies to steal user's focus (attention) and display whatever they were created for. Popup windows are tools that should be used for specific purposes, mostly informative, like tooltips or contextual help.

Galatei wrote:

1. I disagree that the popup window shouldn't steal focus from its parent. Who decided that popup windows shouldn't steal focus? Which windows is stealing focus and which isnt't depends on the situation and specific needs.
In fact, logically thinking, there should be only one control on the screen with focus - this is one of the principles of Windows programming. If you want do allow user to do something in your popup, like select something from combobox (one of the examples), then focus should migrate to that combobox and return it back when the popup is closed.

3. Creating applications that leave users with few controls having focus at the same time on the screen is wrong and misleading.

Galatei wrote:

Imagine disabled people using someone's software designed with such idea in mind, who's having troubles using it, because there's 5 popups on top of the window with focus - which have focus too (that's just one example).

I'd rather urge developers to learn proper UI design techniques, and treat such ideas with some distance, always trying to develop application with all people in mind (which doesn't mean they have to be ugly or not have fancy controls). You can't even imagine how many applications on the market are not suitable for disabled people, or people with little computer use knowledge - believe me - more than 75%.

toolStripDropDownButton.DropDown = new Popup(new Button());