Tooltip widget is made in GTK3 as following tree:
* Tooltip window
* Horizontal Box
* Icon (not supported by Firefox)
* Label
Each element can be fully styled by CSS of GTK theme so we have to draw all
elements with appropriate offset and full box model.
MozReview-Commit-ID: E9yYd5UWBu4
The Gtk 3.20 scrollbars has moved towards usual box model. The scrollbar,
trough,thumb and scrollbar button can now have margin, padding and border set,
different for each direction (ie. left, right, bottom, top). The scrollbar
metrics become ignored in Gtk 3.20 and later.
* Draw scrollbar element [for GTK 3.20+]
* The border for scrollbar, trough, thumb and scrollbar buttons is newly
calculated as margin+padding+border [for GTK 3.20+].
* The margin is subtracted for scrollbar, trough and sb buttons during paint
function [for GTK 3.6+]
* All scrollbar widget's borders transfered from
nsNativeThemeGTK::GetWidgetBorder to the moz_gtk_get_widget_border.
* Added helper function NativeThemeToGtkTheme for mapping mozilla's widget type
to the gtk widget type.
* Scrollbar troughs are now drawn even when there is not enough room for
the thumb [GTK 3.20+]
MozReview-Commit-ID: jd2q67gKM1
Inner borders and focus theming has been removed in Gtk 3.14 so we can't use them to calculate button size.
The size should be computed as border + padding.
