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In the previous commit, we expanded layout viewport height but during reflow the expanded layout viewport size hasn't reflected in ScrollReflowInput::mContentsOverflowAreas.ScrollableOverflow(). We explicitly need to use the expanded height to tell whether we need vertical vertical scrollbars or not. Note that the expanded layout viewport height should NOT be used for non-overlay scrollbars cases since, for example, if the content width is specified by `width: 100%`, the non-overlay vertical scrollbar narrows down the content's used width a little bit because of the vertical scrollbar width, which in turn the minimum scale gets bigger because the content's width became bit narrower, thus the layout viewport size calculated with the minimum scale gets smaller, then it results the vertical scrollbar no longer needs to be rendered, thus the minimum scale gets back to the original value, then the vertical scroll needs to be rendered again, thus this sequence of processes happens repreatedly. Differential Revision: https://phabricator.services.mozilla.com/D40772 --HG-- extra : moz-landing-system : lando