C#

namespace focus_test
{
    public partial class Form1 : Form
    {
        int xOffset, yOffset;
        bool buttonClicked;
        public Form1()
        {
            xOffset = yOffset = 0;
            buttonClicked = false;
            InitializeComponent();
        }

        private void button1_MouseDown(object sender, MouseEventArgs e)
        {
            xOffset = e.X;
            yOffset = e.Y;
            buttonClicked = true;

            if (button1.Parent != this)
            {
                button1.Parent = this;
                button1.Left += panel1.Left;
                button1.Top += panel1.Top;
            }            
            button1.BringToFront();
        }

        private void button1_MouseUp(object sender, MouseEventArgs e)
        {
            buttonClicked = false;
        }

        private void button1_MouseMove(object sender, MouseEventArgs e)
        {
            if (buttonClicked)
            {
                button1.Left += (e.X - xOffset);
                button1.Top += (e.Y - yOffset);
            }
        }
    }
}