Orientation change setection only works from Portrait to Landscape

[Jmaes Wang]

I need to detect mobile browser screen orientation change. It's interesting that the following javascript can detect the orientation change from portrait to landscape but not the opposite. Why?

<script type='text/javascript'>
    detectOrientation();
    window.onorientationchange = detectOrientation;
    function detectOrientation() {
        var panel2CleinetID = '<%= Panel2.ClientID %>';
        var panel2 = document.getElementById(panel2CleinetID);
        var panel3CleinetID = '<%= Panel3.ClientID %>';
        var panel3 = document.getElementById(panel3CleinetID);
 
        //if (typeof window.onorientationchange != 'undefined') {
        if (window.orientation) {
            if (orientation == 0) {
                //Do Something In Portrait Mode
                if (panel2) {
                    panel2.style.display = 'inherit';
                }
                if (Panel3) {
                    panel3.style.display = 'none';
                }
                //alert('orientation == 0');
            }
            else if (orientation == 90) {
                //Do Something In Landscape Mode
                if (panel2) {
                    panel2.style.display = 'none';
                }
                if (Panel3) {
                    panel3.style.display = 'inherit';
                }
                //alert('orientation == 90');
            }
            else if (orientation == -90) {
                //Do Something In Landscape Mode
                if (panel2) {
                    panel2.style.display = 'none';
                }
                if (Panel3) {
                    panel3.style.display = 'inherit';
                }
                //alert('orientation == -90');
            }
            else if (orientation == 180) {
                //Do Something In Portrait Mode
                if (panel2) {
                    panel2.style.display = 'inherit';
                }
                if (Panel3) {
                    panel3.style.display = 'none';
                }
                //alert('orientation == 180');
            }
            else {
                // Get the screen size and determine if it is in portrait
                // or landscape mode by comparing the width and height.
                var wid = screen.width;
                var ht = screen.height;
                if (wid > ht) {
                    if (panel2) {
                        panel2.style.display = 'none';
                    }
                    if (Panel3) {
                        panel3.style.display = 'inherit';
                    }
                    //alert('wid > ht');
                }
                else {
                    if (panel2) {
                        panel2.style.display = 'inherit';
                    }
                    if (Panel3) {
                        panel3.style.display = 'none';
                    }
                    //alert('wid < ht');
                }
            }
        }
    }
</script>