It turns out that any attempt to set table margins in stylesheet code will cause this problem. For example,
table
{
margin-left: auto;
margin-right: auto;
}
also causes the problem. However, I did find a solution, and that is to encapsulate each drop-down menu into their own group, as shown below:
<item type="g_start" />
<item type="dropdown" name="FormatBlock" width="70" RenderItemBorder="true" text="[[Paragraph]]"command="FormatBlock" />
<item type="g_end" />
<item type="g_start" />
<item type="dropdown" name="FontName" width="110" text="[[FontName]]" command="FontName" />
<item type="g_end" />
<item type="g_start" />
<item type="dropdown" name="FontSize" width="60" text="[[FontSize]]" command="FontSize" />
<item type="g_end" />
By setting a g_start and g_end around each dropdown, the auto margins do not expand to fill the available space. This does take slightly more horizontal menu room, but not much.