question an eventuality perculiarity tabcontrol

i have small explanation next rare problem.

using system;
using system.windows.forms;

namespace windowsapplication1
{
    open category testform : form
    {
        private system.windows.forms.tabcontrol tabcontrol1;
        private system.windows.forms.tabpage tabpage1;
        private system.windows.forms.tabpage tabpage2;
        private system.windows.forms.textbox textbox1;

        open testform()
        {
            //controls
            this.tabcontrol1 = new system.windows.forms.tabcontrol();
            this.tabpage1 = new system.windows.forms.tabpage();
            this.tabpage2 = new system.windows.forms.tabpage();
            this.textbox1 = new system.windows.forms.textbox();

            // tabcontrol1
            this.tabcontrol1.anchor = ((system.windows.forms.anchorstyles)((((system.windows.forms.anchorstyles.top | system.windows.forms.anchorstyles.bottom)
                        | system.windows.forms.anchorstyles.left)
                        | system.windows.forms.anchorstyles.right)));
            this.tabcontrol1.controls.add(this.tabpage1);
            this.tabcontrol1.controls.add(this.tabpage2);
            this.tabcontrol1.location = new system.drawing.point(12, 12);
            this.tabcontrol1.name = "tabcontrol1";
            this.tabcontrol1.selectedindex = 0;
            this.tabcontrol1.size = new system.drawing.size(260, 240);
            this.tabcontrol1.tabindex = 0;
            this.tabcontrol1.selected += new system.windows.forms.tabcontroleventhandler(this.tabcontrol1_selected);

            // tabpage1
            this.tabpage1.controls.add(this.textbox1);
            this.tabpage1.location = new system.drawing.point(4, 22);
            this.tabpage1.name = "tabpage1";
            this.tabpage1.size = new system.drawing.size(252, 214);
            this.tabpage1.tabindex = 0;
            this.tabpage1.text = "tabpage1";

            // tabpage2
            this.tabpage2.location = new system.drawing.point(4, 22);
            this.tabpage2.name = "tabpage2";
            this.tabpage2.size = new system.drawing.size(192, 74);
            this.tabpage2.tabindex = 1;
            this.tabpage2.text = "tabpage2";

            // textbox1
            this.textbox1.anchor = ((system.windows.forms.anchorstyles)(((system.windows.forms.anchorstyles.top | system.windows.forms.anchorstyles.left)
                        | system.windows.forms.anchorstyles.right)));
            this.textbox1.location = new system.drawing.point(6, 38);
            this.textbox1.name = "textbox1";
            this.textbox1.size = new system.drawing.size(240, 20);
            this.textbox1.tabindex = 0;

            // testform
            this.clientsize = new system.drawing.size(284, 264);
            this.controls.add(this.tabcontrol1);
            this.name = "form1";
            this.text = "form1";
        }

        //tab selected
        private vacant tabcontrol1_selected(object sender, eventargs e)
        {
            this.text = "textbox width: " + this.textbox1.width.tostring();
        }
    }

    //main
    stationary category program
    {
        stationary vacant main()
        {
            application.run(new testform());
        }
    }

}

if run above c# formula have little form containing tabcontrol. within tabcontrol texbox initial tab. follow stairs problem:

1. select tabpage2 (textbox1's extent reported form title)


2. resize form


3. select tabpage1 (the wrong textbox1 extent reported)

any ideas going here? textbox apparently bigger being reported. click again tabpage2 repremand distance following updated. apparently there an eventuality updating extent textbox1. i trigger tabpage1 selected?