question

@LooBi, Welcome to Microsoft Q&A, you could adjuest two panel's size and location to expand or reduce the two panels.

Here is a code example you could refer to.

    Panel panel1 = new Panel();
         Panel panel2 = new Panel();
         private void Form1_Load(object sender, EventArgs e)
         {
              
             panel1.BackColor = Color.Blue;
             panel1.Size=new Size(tabPage1.Width,tabPage1.Height/2);
             panel1.Click += Panel1_Click;
             TextBox textBox1 = new TextBox();
             textBox1.Text = "Hello";
             panel1.Controls.Add(textBox1);
    
             panel2.BackColor = Color.Yellow;
             panel2.Size = new Size(tabPage1.Width, tabPage1.Height / 2);
             panel2.Location = new Point(panel1.Left, panel1.Bottom);
             panel2.Click += Panel2_Click;
             CheckBox checkBox1 = new CheckBox();
             checkBox1.Text = "Do you agree";
             checkBox1.Checked = false;
             panel2.Controls.Add(checkBox1);
             tabPage1.Controls.Add(panel1);
    
             tabPage1.Controls.Add(panel2);
         }
    
         private void Panel2_Click(object sender, EventArgs e)
         {
             panel1.Size = new Size(tabPage1.Width, (tabPage1.Height / 3));
             panel2.Size = new Size(tabPage1.Width, (tabPage1.Height / 3)*2);
             panel2.Location = new Point(panel1.Left, panel1.Bottom);
         }
    
         private void Panel1_Click(object sender, EventArgs e)
         {
             panel1.Size = new Size(tabPage1.Width, (tabPage1.Height / 3) * 2);
             panel2.Size = new Size(tabPage1.Width, (tabPage1.Height / 3));
             panel2.Location = new Point(panel1.Left, panel1.Bottom);
    
         }

Result:

As the above picture shown, when I click panel1, panel1 will expand and panel2 will reduce, so on the panel2.

Best Regards,
Jack

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