When safe at startup to get element positions/dimensions?

I have a canonical ("extends Composite") GWT 2.5.1/UiBinder app. There are three horizontal sections on the browser page, comprised of div elements; let's call them top, middle, and bottom. The layout is fluid/dynamic with respect to the browser's window size. I have Java code that vertically centers the middle section in the space between the top and bottom sections; I do this at startup, and also on a browser resize event, by calling this method:

    protected void positionMiddle() {
        int topOfSpace = topSection.getAbsoluteBottom();
        int botOfSpace = botSection.getAbsoluteTop();
        int middleHeight = middleSection.getClientHeight();
        int topOfMiddle = topOfSpace + (botOfSpace - topOfSpace - middleHeight)/2;
        DOM.setStyleAttribute(DOM.getElementById("midSection"), "top", topOfBoard + "px");
    }

If startup call is done in my app/widget's constructor, or if it is done in an onLoad() method, it doesn't work correctly because getAbsoluteBottom, getAbsoluteTop, and getClientHeight all return 0. What I have done is wait until the first user interaction, which must be a button click to dismiss a splash screen, to call the above method; that works fine. I reason that the method call must be delayed until the browser has finished its rendering, i.e., finished its layout computations. I am wondering whether there is some way, such as an event I could detect via a handler, to know when that is. While having the first call to positionMiddle() in a button click handler works OK, it seems inelegant. Also, I'd like to better understand the relation between GWT and browser rendering at startup.

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