I was running into a problem with my context menu showing up about 200px below my cursor. The FlyTreeView is in a user control that is in a user control that is in a user control that is in a page (some fun nesting!). It seemed like the context menu was showing up the same amount below my cursor that the innermost control was from the top of the page. On Firebug inspection, the rendered table containing the context menu had a position of absolute, so when the top style was set, it was going from the top of the control -- not the top of the page. A position of fixed would be required to achieve the best effect. Here's my hack:
style type="text/css">
/* FlyTreeView's context menu by default has an absolute positioning
* This is fine if we're weren't already in a nested control. Instead
* It needs a fixed positioning
*/
#uglyHack table
{
position: fixed !important;
}