XUL Panel triggered by SELECT element is cut off at the end

Hi.

I have a panel that I show whenever I hover specific inputs in a page.

This works well in almost every input element, but on SELECT elements, the panel is cut at the end of the select element:

(screenshot: http://i.stack.imgur.com/hNfzK.png)

I open the panel with:

panel.openPopup(node, "before_start", 0, 0, false, false);

Is there some specificity to the SELECT element that i'm missing?