private void beginDragging(final MouseDownEvent event) {
        getDragGlass().show();

        dragging = true;
        dragStartX = event.getClientX();
        dragStartY = event.getClientY();
        dragStartWindow = new Rect(getElement());
        DOM.setCapture(getElement());
    }
Called on mouse down in the caption area, begins the dragging loop by
turning on event capture.

@param event the mouse down event that triggered dragging
@see DOM#setCapture
@see #continueDragging
stroom-core-client-widget/src/main/java/stroom/widget/popup/client/view/Dialog.java