We have the following scenario:
<tabbox hflex=1 vflex=1 orient=left>
<tabs width=20%>
<!-- some tabs -->
</tabs>
<tabpanels hflex=1 vflex=1>
<tabpanel hflex=1 vflex=1>
<!-- some content with overflow:auto; hflex == vflex == 1 -->
</tabpanel>
</tabpanels>
</tabbox>

While this works great on bigger resolutions, I noticed that the Tabpanels element is moved below my viewport when I resize to a smaller window.
When I inspect its DOM element with the developer tools, and remove the inline width that's set by hflex, it is shown again.
Alternately, if I change the inline width to one pixel less, it is also displayed again.

This may be due to the cases where the percentage width set on the Tabs gives out a decimal pixel value which is ceil'd by the browser, instead of floor'd.
For an experiment, you can try setting decimal pixel widths in the inline style. Notice that 0.1px is enough to break it.