Dynamically-sized control with float:right width

I'm running into a problem that seems like it must be common, but I haven't found any answers yet.
 I'm trying to design a small pop-up window styled panel, which consists of a title bar div with some text and a right-aligned ImageButton.  Underneath the title bar is some dynamically-sized content, such as a GridView.   With my current code, the ImageButton is not displayed at the far right unless the GridView is really small.  I realize the problem is that it doesn't know at render time how big the GridView is going to be.  So my question is whether there is a way to accomplish this other than using javascript to resize the title bar div when the dynamic content loads?  Here's a simplified version of my code:
<div style="position: relative">
            <div style="position: absolute">
       ...