UI rect touch is not working?

1. I create a new project

2. I create a canvas

3. I create a panel

4. I change the panel rect transform in the inspector to stretch

So, it looks like image 1.

Then I add t$$anonymous$$s code to the UI panel.

 private bool checkFirstTouch = true;

 void OnEnable()
 {
     checkFirstTouch = true;

 }

 void Update () {
     
     if (checkFirstTouch) {
     
         if (Input.GetMouseButton (0)) {    
             
             foreach (Touch touch in Input.touches) {
                 
                 if (gameObject.GetComponent<RectTransform>().rect.Contains(touch.position)) {
                     print ("Touched.");
                     
                     checkFirstTouch = false;
                 }
             }
             
         }            
         
     }

 }

My problem here is, that when I touch the screen on my mobile device, it's only working when I touch the left bottom corner. Please look image 2.

Why?

Why the whole screen is not working? Because the panel is stretched?

Must I multiply it with canvas scaler or what?

My canvas looks like t$$anonymous$$s:

[Canvas]: Screen Space Overlay

[CanvasScaler]:

UI Scale Mode: Scale with Screen Size

Reference Resolution: X: 800, Y: 480

Screen Match Mode: Match Width Or Height

Match: 0.5

I'm using Unity 4.7.0.

EDIT:

If I print "gameObject.GetComponent ().rect", I get t$$anonymous$$s:

(x:-400.00, y:-240.00, width:800.00, height:480.00)