Bug 1464723 - Implement custom scrollbar support for GTK widget. r=karlt,dholbert

When any scrollbar color is specified, or scrollbar-width is thin, we
switch to use the fallback rendering.

The change to xulscrollbars.css is for ensuring that the scrollbar is
displayed for scrollbar-width: thin when there is no scrollbar color
specified. It wouldn't affect cases where -moz-appearance takes effect.

This also changes the fallback width of the scrollbars. Since the two
widths was picked rather randomly, I think it should be fine to change
it if a value looks better than the old one, especially on Linux which
is the main usecase for this fallback rendering.

Differential Revision: https://phabricator.services.mozilla.com/D3952

--HG--
extra : moz-landing-system : lando

layout/xul/nsBox.cpp

@@ -532,9 +532,9 @@ GetScrollbarWidthNoTheme(nsIFrame* aBox)
     switch (scrollbarStyle->StyleUIReset()->mScrollbarWidth) {
       default:
       case StyleScrollbarWidth::Auto:
-        return 16 * AppUnitsPerCSSPixel();
+        return 12 * AppUnitsPerCSSPixel();
       case StyleScrollbarWidth::Thin:
-        return 8 * AppUnitsPerCSSPixel();
+        return 6 * AppUnitsPerCSSPixel();
       case StyleScrollbarWidth::None:
         return 0;
     }

toolkit/themes/windows/global/xulscrollbars.css

@@ -14,6 +14,7 @@
 scrollbar {
   -moz-appearance: scrollbar-horizontal;
   cursor: default;
+  background-color: #f0f0f0;
 }
 
 @media all and (-moz-overlay-scrollbars) {
@@ -43,6 +44,10 @@ slider[orient="vertical"] {
 
 /* ::::: thumb (horizontal) ::::: */
 
+thumb {
+  background-color: #cdcdcd;
+}
+
 thumb[orient="vertical"] {
   -moz-appearance: scrollbarthumb-vertical;
   min-height: 8px;

widget/gtk/nsNativeThemeGTK.cpp

@@ -1866,6 +1866,15 @@ nsNativeThemeGTK::ThemeSupportsWidget(nsPresContext* aPresContext,
   if (IsWidgetTypeDisabled(mDisabledWidgetTypes, aWidgetType))
     return false;
 
+  if (IsWidgetScrollbarPart(aWidgetType)) {
+    ComputedStyle* cs = nsLayoutUtils::StyleForScrollbar(aFrame);
+    if (cs->StyleUI()->HasCustomScrollbars() ||
+        // We cannot handle thin scrollbar on GTK+ widget directly as well.
+        cs->StyleUIReset()->mScrollbarWidth == StyleScrollbarWidth::Thin) {
+      return false;
+    }
+  }
+
   if (aWidgetType == StyleAppearance::MenulistButton &&
       StaticPrefs::layout_css_webkit_appearance_enabled()) {
     aWidgetType = StyleAppearance::Menulist;