VS 2008 - JavaScript GetClientRects Does Not Work In Content Page

I have a aspx page with a table with scrollable rows..I achieved this by having a "header" table, and a "rows" table. The rows table has a div with "over flow:auto" so get the scroll bar. When data is loaded in the rows table, the columns have different sizes, and since the columns are not fixed width, the header columns don't match with rows columns.

using JavaScript, I loop through the first row in the "rows" table, and I call the getClientRects() function to get the cell bounds, calculate the column width and assign that value to the header table columns. This works perfectly when I use a regular aspx page. I decided to switch to a master-content architecture, because I have a tab menu at the top of the page, and I copied and pasted the code into the content page. Everything works fine, but the getClientRects() returns all values as 0 (zeros). So how can I get the getClientRects() to return the propper values ?Here is the relevant code:

ASP Code:

<script language="JavaScript" type="text/javascript">   
function SetTableColWidth() {       
var th = document.getElementById("tbl_SearchResult_Header");       
var tr = document.getElementById("tbl_SearchResult_Rows");        
if (tr != null) {     

[code]....