Изменение ширины окна влево

    public partial class Form1 : Form
    {
        [System.Runtime.InteropServices.DllImport("user32.dll", SetLastError = true)]
        private static extern bool MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);
 
        private int ResizeWidth = 30;
 
        public Form1()
        {
            InitializeComponent();
            MouseEnter += Form1_MouseEnter;
            MouseLeave += Form1_MouseLeave;
        }
        
        private void Form1_MouseEnter(object sender, EventArgs e)
        {
            MoveWindow(Handle, Location.X - ResizeWidth, Location.Y, Width + ResizeWidth, Height, true);
        }
 
        private void Form1_MouseLeave(object sender, EventArgs e)
        {
            MoveWindow(Handle, Location.X + ResizeWidth, Location.Y, Width - ResizeWidth, Height, true);
        }
    }