Ah, right, forgot about the resize, sorry. You'll have to remove the
resize="true" from the outermost box, and give it a size (e.g. width="125?pagew" height="24"). You should also remove the wrapping vflow, I think. So basically it'll become:
<box id="zoomframe" style="container" x="25" y="25" mouseenabled="true" alpha_out="0.3" width="125?pagew" height="24">
<!-- Zoom out -->
<button name="btn_zoom_out" title="localize(LNG_ZOOM_OUT, Zoom out)" style="common zoom_out"/>
<scrollbar name="zoom_slider" anchors="25,5,pw-25,19" background="gradient(linear-horizontal,0x666666,0x4d4d4d)">
<button name="$scroll_handle$" width="15" height="15" style="scroll_handle" title="localize(LNG_ZOOM_SLIDE, Slide to control zoom level)"/>
</scrollbar>
<!-- Zoom in -->
<button name="btn_zoom_in" title="localize(LNG_ZOOM_IN, Zoom in)" style="common zoom_in" anchors="pw-w"/>
</box>