The RadPane'sShowContentDuringLoad property works
along with the ContentUrl one and it controls whether RadPane
will show the page while it is being loaded or will wait until the page is completely
loaded (e.g. window.onload is fired in the content page). If ShowContentDuringLoad
is set to false, RadPane will display a loading image that can
be controlled with CSS.
The RadPane has built-in ability to show a loading sign while a
content page set through the ContentUrl property is being loaded
in it. To turn on this functionality you should set ShowContentDuringLoad
to false (the default value is true).