Xul Scrollbox button disappearing

If you run this simple Xul example, you can see that when you scroll from the last (3°) button to the first, using tab, the middle button disappear. Any idea why? And how to fix?

(to run it you'll need to put an png image named "img.png" at content folder)

test.xul

<?xml-stylesheet href="chrome://test/content/test.css" type="text/css"?>

<window id="desktop" title="test" width="1280" height="720" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

    <scrollbox>
        <button image="chrome://test/content/img.png" />
        <button image="chrome://test/content/img.png" />
        <button image="chrome://test/content/img.png" />
    </scrollbox>

</window>

test.css

#desktop {
    border-style:solid;
    border-color:transparent;
    border-width:50px;
}

button {
    position: absolute;
    -moz-appearance: none;
    border: none;
    background: url(""); /* nothing */
    width: 128px;
    height: 128px;
    z-index: 1;
}

button image {
    width: 64px;
    height: 64px;
}

button:focus image {
    width: 128px;
    height: 128px;
}

scrollbox {
    background-color: #555555;
}

Solution

Solved inserting the scrollbox inside a box, and setting the scroll's background color to the box.