To reproduce:

 1. drag a radgroupbox into form
 2. drag 3 radpanels into radgroupbox
 3. set the 3 radpanels' dock = dockstyle.top
 4. set radgroupbox. autosize = true
 5. drag a radbutton into form and out of radgroupbox
 6. add click eventhandler to radbotton with below code

private void Button1_Click(object sender, EventArgs e)
{
       RadPanel2.Visible = !RadPanel2.Visible;
}

7. run app,  click the button continuously,  you will see that the width of the RadGroupBoxwill increase.

Workaround:

Set the AutoSize property to false and prior to hiding/showing the panel modify the size of the RadGroupBox