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
