my problem with scrollHeight

Question

 I have a position:fixed <aside> navigation sidebar:

<ul> 

<li>

<a onclick="showChildren()" >item</a>

    <ul style="display: none;">

    <li>

        some content...

    </li>

</ul>

</li>

</ul> 

 

when I scroll bottom it's fine, but if I click one item, the children ul will display, and the displayed ul will overlay on the bottom, the ul part can't be scrolled.

 

How I can make the ul display and increase the scroll height?