I need to modify the CMSTabControl in 5.5, but any change that I make does not appear on my web page in CMS Desk in any viewing mode.

Here is the file I am trying to change...
E:\KenticoCMS\CMSWebParts\Navigation\cmstabcontrol.ascx.cs

Here is the most simple example of a change that is not visible...
protected override void OnPreRender(EventArgs e)
{
base.OnPreRender(e);
this.Visible = this.tabElem.Visible;

...(other standard code that comes with the control)...

// test code
this.Visible = false;
}

I expected the addition of the test code line to cause the menu to be invisible, yet I can still see it. Suggestions?