Reused GWT Widget becomes not visible - gwt

Question

I have Widget with a TabBar.
<g:DockLayoutPanel unit="EM">
<g:north size="2.5">
<g:TabBar height="2.5em" ui:field="tabBar" />
</g:north>
<g:center>
<g:SimpleLayoutPanel ui:field="contentWidget" />
</g:center>
</g:DockLayoutPanel>
If user selects a tab, I set the widget for contentWidget.
#UiHandler("tabBar")
void onTabsSelection(final SelectionEvent<Integer> event) {
// ...
}
This works perfectly!
Several tabs have ONE Widget in common, so I want to use a single instance of this widget (for sync).
<g:LayoutPanel>
<!-- query -->
<g:layer top="0" height="3em">
<g:HorizontalPanel width="100%" height="3em" verticalAlignment="ALIGN_MIDDLE">
<g:SimpleLayoutPanel ui:field="queryNode" />
<g:cell horizontalAlignment="ALIGN_RIGHT">
<my:StepPager ui:field="pager" />
</g:cell>
</g:HorizontalPanel>
</g:layer>
<!-- map -->
<g:layer left="0" right="17em" top="3em" bottom="0">
<g:SimpleLayoutPanel ui:field="mapNode" />
</g:layer>
</g:LayoutPanel>
Of course a widget can only be attached to one parent. So I override the onAttach method for all the tab-wigets to read my query-widget.
#Override
protected void onAttach() {
queryNode.setWidget(queryBar);
super.onAttach();
}
The method will be invoked and the widget is in the DOM but it's not visible. I think the problem is that the cell of SimpleLayoutPanel queryNode size is 3x35 px.