private void rtbSzoveg_DragEnter(object sender, DragEventArgs e)
{
    e.Effect = e.Data.GetDataPresent(DataFormats.FileDrop) ? DragDropEffects.Copy : DragDropEffects.None;
}

private void rtbSzoveg_DragDrop(object sender, DragEventArgs e)
{
    string[] fajlok = (string[])e.Data.GetData(DataFormats.FileDrop);
    string fajl = fajlok[0];

    if (System.IO.Path.GetExtension(fajl).ToLower() != ".txt")
    {
        MessageBox.Show("Csak TXT fájlt lehet ejteni.");
        return;
    }

    try
    {
        rtbSzoveg.Text = System.IO.File.ReadAllText(fajl);
    }
    catch (Exception ex)
    {
        MessageBox.Show("A fájl nem olvasható: " + ex.Message);
    }
}

private int bal = 0;
private int jobb = 0;
private int kozep = 0;

private void pnlTer_MouseClick(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        bal = bal + 1;
        lblBal.Text = "Bal: " + bal;
    }
    else if (e.Button == MouseButtons.Right)
    {
        jobb = jobb + 1;
        lblJobb.Text = "Jobb: " + jobb;
    }
    else if (e.Button == MouseButtons.Middle)
    {
        kozep = kozep + 1;
        lblKozep.Text = "Közép: " + kozep;
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    pbLogo.Cursor = Cursors.Hand;
}

private void pbLogo_Click(object sender, EventArgs e)
{
    System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
    {
        FileName = "https://www.digitalisfelfedezok.hu",
        UseShellExecute = true
    });
}

private void Cimke_MouseEnter(object sender, EventArgs e)
{
    Label cimke = (Label)sender;
    cimke.Tag = cimke.BackColor;      // az eredeti szín megjegyzése
    cimke.BackColor = Color.Yellow;
}

private void Cimke_MouseLeave(object sender, EventArgs e)
{
    Label cimke = (Label)sender;

    if (cimke.Tag is Color eredeti)
    {
        cimke.BackColor = eredeti;
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    tmrKattintas.Interval = SystemInformation.DoubleClickTime;
}

private void pnlGomb_Click(object sender, EventArgs e)
{
    tmrKattintas.Stop();
    tmrKattintas.Start();   // az egyszeri kattintás eredménye késik
}

private void tmrKattintas_Tick(object sender, EventArgs e)
{
    tmrKattintas.Stop();
    lblEredmeny.Text = "Egy";
}

private void pnlGomb_DoubleClick(object sender, EventArgs e)
{
    tmrKattintas.Stop();
    lblEredmeny.Text = "Kettő";
}

private bool mozog = false;
private Point kezdoEger;
private Point kezdoAblak;

private void pnlCim_MouseDown(object sender, MouseEventArgs e)
{
    mozog = true;
    kezdoEger = Cursor.Position;
    kezdoAblak = this.Location;
}

private void pnlCim_MouseMove(object sender, MouseEventArgs e)
{
    if (!mozog)
    {
        return;
    }

    this.Location = new Point(
        kezdoAblak.X + Cursor.Position.X - kezdoEger.X,
        kezdoAblak.Y + Cursor.Position.Y - kezdoEger.Y);
}

private void pnlCim_MouseUp(object sender, MouseEventArgs e)
{
    mozog = false;
}

private ListBox forras = null;

private void Lista_MouseDown(object sender, MouseEventArgs e)
{
    ListBox lista = (ListBox)sender;
    int index = lista.IndexFromPoint(e.Location);

    if (index == ListBox.NoMatches)
    {
        return;
    }

    forras = lista;
    lista.DoDragDrop(lista.Items[index].ToString(), DragDropEffects.Move);
}

private void Lista_DragEnter(object sender, DragEventArgs e)
{
    e.Effect = e.Data.GetDataPresent(DataFormats.Text) ? DragDropEffects.Move : DragDropEffects.None;
}

private void Lista_DragDrop(object sender, DragEventArgs e)
{
    ListBox cel = (ListBox)sender;

    if (cel == forras)
    {
        return;
    }

    string elem = (string)e.Data.GetData(DataFormats.Text);
    cel.Items.Add(elem);
    forras.Items.Remove(elem);
}

private void Form1_Load(object sender, EventArgs e)
{
    rtbSzoveg.MouseWheel += rtbSzoveg_MouseWheel;
}

private void rtbSzoveg_MouseWheel(object sender, MouseEventArgs e)
{
    if (Control.ModifierKeys != Keys.Control)
    {
        return;
    }

    float uj = rtbSzoveg.ZoomFactor + (e.Delta > 0 ? 0.1f : -0.1f);
    rtbSzoveg.ZoomFactor = Math.Max(0.5f, Math.Min(3f, uj));
}