private Element getGlass() {
        if (glassElem == null) {
            glassElem = DOM.createDiv();
            glassElem.getStyle().setPosition(Position.ABSOLUTE);
            glassElem.getStyle().setLeft(0, Unit.PX);
            glassElem.getStyle().setTop(0, Unit.PX);
            glassElem.getStyle().setMargin(0, Unit.PX);
            glassElem.getStyle().setPadding(0, Unit.PX);
            glassElem.getStyle().setBorderWidth(0, Unit.PX);
            glassElem.getStyle().setZIndex(2);

            // We need to set the background color or mouse events will go right through the glassElem. If the
            // SplitPanel contains an iframe, the iframe will capture the event and the slider will stop moving.
            glassElem.getStyle().setBackgroundColor("white");
            glassElem.getStyle().setOpacity(0);
            glassElem.getStyle().setProperty("filter", "alpha(opacity=0)");
        }

        return glassElem;
    }
Sets the minimum allowable size for the given widget.
<p>
<p>
Its associated splitter cannot be dragged to a position that would make
it smaller than this size. This method has no effect for the
{@link DockLayoutPanel.Direction#CENTER} widget.
</p>

@param child   the child whose minimum size will be set
@param minSize the minimum size for this widget
stroom-core-client-widget/src/main/java/com/google/gwt/user/client/ui/MySplitLayoutPanel.java