As mousemove inside an iframe does not get handled through to the 'surrounding' page, I tried to relay that event using a bit of a code found on StackOverflow:
https://stackoverflow.com/questions/...over-an-iframe
Code:
function bubbleIframeMouseMove(iframe){
    iframe.contentWindow.addEventListener('mousemove', function( event ) {
        var boundingClientRect = iframe.getBoundingClientRect();

        var evt = new CustomEvent('mousemove', {bubbles: true, cancelable: false});
        evt.clientX = Math.floor(event.clientX + boundingClientRect.left);
        evt.clientY = Math.floor(event.clientY + boundingClientRect.top);

       iframe.dispatchEvent(evt);

    });

};
and since IE does not know CustomEvent, there's a polyfill for IE as well:
Code:
(function () {

    if ( typeof window.CustomEvent === "function" ) return false;

    function CustomEvent ( event, params ) {
        params = params || { bubbles: false, cancelable: false, detail: undefined };
        var evt = document.createEvent( 'CustomEvent' );
        evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
        return evt;
    }

    CustomEvent.prototype = window.Event.prototype;

    window.CustomEvent = CustomEvent;
})();
The issue is that this works perfectly fine in firefox, and it does also work fine in IE, but just as long as you don't use Ext.on() or addListener().
This WILL work above the page and above the iframe:
Code:
window.addEventListener('mousemove', function( event ) {
    console.log('dom mouse moved: ' + event.clientX + 'x' + event.clientY);
});
This WILL work above the page but WILL NOT work above the iframe:
Code:
Ext.get(window).on('mousemove', function( event ) {
    console.log('ext mouse moved: ' + event.clientX + 'x' + event.clientY);
});
I tried to use Ext.get(iframe).fireEvent('mousemove', evt) instead of dispatchEvent, or using Ext.get(iframe).relayEvents(Ext.get(iframe.contentWindow), ['mousemove'])
Both did not work at all, not even for addEventListener()

In fact, this did not work either: (also not if you substitute contentDocument)
Code:
Ext.get(domIframe.contentWindow).on('mousemove', function( event ) {
    console.log('inner: ' + event.clientX + 'x' + event.clientY);
});
Complete fiddle for this, don't forget to enable console: https://fiddle.sencha.com/#view/editor&fiddle/29do
Will work fine in firefox, will show the error in IE

Is there any way I can get ext to recognize custom events?

Reason I need this: I got a splitter next to an Iframe. In Ext.dd.DragTracker.onMouseMove(), it will not call triggerStart() until the mouse moved at least 5px from the initial position. Since mouse movement is not reported over an Iframe, that means I can only drag the splitter one way (or drag it away from the iframe and then drag it back over it, once started it seems to work). Using the event relaying above works in firefox, but not in IE, which will be because Ext.on() does not work in that case on IE.