In print preview, if you have "FIT_PAGE_WIDTH" as a default scale mode, once you click the "toggle full page/full page width" button, it's never possible to get it to return the original fit-page-width size (it seems to cycle between 3 different zoom modes, but none of them fill the entire control width).
It looks like the report viewer correctly sets the scale mode during initialization. However, when toggling the mode later, it sets it incorrectly.