Are you talking about Netscape 4.x or 6 and above. I know 4.x has a window resize bug. Netscape also hides the vertical scroll bar if there isn't enough content height-wise to require vertical scrolling.
That is, if the window is resized to a larger size, and there isn't enough content to warrant scrolling down, the vertical scrollbars will disappear and the page will rerender to accomodate the extra width.
This sort of thing happens on NS 6, 7, and Mozilla. Maybe Opera too.
I wonder if you could set the position of the DIV using right: XX%;
That way it would position the DIV from the right, instead of from the left.