This is solved:
The reason was that IFrameCommunicator didn't register the message event listener, until the iframe was loaded instantly.
The default iframe HTML code is:
<div id="iframe_holder" class="center-block" style="width:90%;max-width: 1000px">
<iframe id="load_payment" class="embed-responsive-item" name="load_payment" width="100%" height="650px" frameborder="0" scrolling="yes" hidden="true">
</iframe>
And later JS code is:
$("#iframe_holder iframe").hide();which sets
display:none
on the iframe. Removing this JS code and hidden="true" from iframe inline, the iframe now loads instantly and registers eventListener correctly. Without that, only this code:
if (window.location.hash && window.location.hash.length > 1) {
callParentFunction(window.location.hash.substring(1));
}from IFrameCommunicator would run, which for some reason doesn't include the respone JSON.
