Today I was adding the new HTML Editor control (newly added to the AJAX Control Toolkit) to an Intranet page. Whenever I would mouse click into the HTML Editor it would continue to expand. This would only occur in IE 8. When using Firefox or IE 8 in compatibility mode it wouldn't happen. I tracked it down to the following declaration in my CSS file:
I removed this declaration and instead used the cellpadding attribute for the table. This solved the issue. Very strange.