When overlay scrollbars are enabled (macOS and Android by default, other platforms if you set the ui.useOverlayScrollbars pref), scrollbars shouldn't take up any layout space. The way this works on the platforms it works on is that nsScrollbarFrame::GetXULMargin sets a negative margin on the scrollbar that is equal to its size. However, it only does that if theme->ThemeSupportsWidget(presContext, this, StyleAppearance::Scrollbar). It seems that Windows (?), Linux and the non-native theme don't return true in this call, so overlay scrollbars incorrectly take up space.

Reuse the AddXULMinSize logic which already deals with all the widget stuff,
non-themed scrollbars, etc.

Backed out changeset be3b005bfc6d (Bug 1619428) for causing reftest failure in contain-size-select-elem-002-ref.html CLOSED TREE

https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&classifiedState=unclassified&fromchange=a38866b381a8c49d852aed28d9dab9a426a70fcc&searchStr=android%2C7.0%2Cx86-64%2Cquantumrender%2Copt%2Creftests%2Ctest-android-em-7.0-x86_64-qr%2Fopt-geckoview-reftest-e10s-1%2Cr%28r1%29&selectedJob=291488460

https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=291488460&repo=autoland&lineNumber=5860

https://hg.mozilla.org/mozilla-central/rev/76f13c8e10b7
https://hg.mozilla.org/mozilla-central/rev/c26f12bfc75c
https://hg.mozilla.org/mozilla-central/rev/58d71374ca38
https://hg.mozilla.org/mozilla-central/rev/07c44cc38c7b