[css-om-view] MouseEventInit should allow pageX/Y

[timdream]

The MouseEvent interface exposes pageX/Y but it doesn't allow the constructor to specify pageX/Y value.

At least on Safari, the pageX/Y value of a constructed MouseEvent (and sub-class instances) will simply be the fix to the value specified in clientX/Y, disregarding the scrolling position of the parent elements when dispatched.

My use case for pageX/Y is to be able to test the event consumption of my own script on the element. I had to result to use something like this:

const rect = targetElement.getBoundingClientRect();
let event = new WheelEvent("wheel", {
        // WheelEventInit
        deltaX: 0,
        deltaY: 4 * (invert ? -1 : 1),
        deltaZ: 0,
        deltaMode: WheelEvent.DOM_DELTA_PIXEL,
        // MouseEventInit
        clientX: rect.x + rect.width / 2,
        clientY: rect.y + rect.height / 2
    });
Object.defineProperty(event, "pageX", { value: rect.x + rect.width / 2 });
Object.defineProperty(event, "pageY", { value: document.scrollingElement.scrollTop + rect.y + rect.height / 2 });
targetElement.dispatchEvent(event);

It will be merrier to be able to specify pageX/Y directly on MouseEventInit.