How to get preferred dimension but without sub effect?

[blp330]

Hi,

After loading a web page, I want to get preferred dimension and resize
window to fit it for getting a snapshot of web page.

I do this to get preferred dimension:

 // ... Get some variables.
 nsresult rv = presShell->ResizeReflow(prefWidth,
NS_UNCONSTRAINEDSIZE);
 NS_ENSURE_SUCCESS(rv, false);

 PRInt32 width, height;
 nsRect shellArea = presContext->GetVisibleArea();
 NS_ENSURE_TRUE(shellArea.width != NS_UNCONSTRAINEDSIZE &&
                                shellArea.height !=
NS_UNCONSTRAINEDSIZE,
                                false);

 width = presContext->AppUnitsToDevPixels(shellArea.width);
 height = presContext->AppUnitsToDevPixels(shellArea.height);

 preferredWidth = width + 1;
 preferredHeight = height;

This works great but I found it will cause the snapshot of
google.com.tw with scrollbar.
Look like this:
http://mercurius.f-snet.com/google-1.png

I try to resize window width to 1024, but it will become:
http://mercurius.f-snet.com/google-2.png

It still has scrollbar.

I found that after I call presShell->ResizeReflow(prefWidth,
NS_UNCONSTRAINEDSIZE);
That scrollbar will appear even I resize the window.

How can I get preferred dimension without such sub effect?

Thank you very much.
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