Bug 1463687 - Apply scrollbar style of root element to scrollbars of viewport. r=heycam

MozReview-Commit-ID: GWmhehtqO1U

--HG--
extra : rebase_source : 76e460a6f57a4caa852ee1ada1c8e213c598bbd1

layout/base/nsLayoutUtils.cpp

@@ -20,6 +20,7 @@
 #include "mozilla/Maybe.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/dom/ContentChild.h"
+#include "mozilla/ServoStyleSetInlines.h"
 #include "mozilla/StaticPrefs.h"
 #include "mozilla/Unused.h"
 #include "nsCharTraits.h"
@@ -10186,3 +10187,27 @@ nsLayoutUtils::ParseFontLanguageOverride(const nsAString& aLangTag)
   }
   return result;
 }
+
+/* static */ ComputedStyle*
+nsLayoutUtils::StyleForScrollbar(nsIFrame* aScrollbarPart)
+{
+  nsIFrame* scrollFrame =
+    GetClosestFrameOfType(aScrollbarPart, LayoutFrameType::Scroll);
+  MOZ_ASSERT(scrollFrame, "Scrollbar part frame "
+             "should have a scroll frame ancestor");
+
+  if (scrollFrame->GetParent()->IsViewportFrame()) {
+    nsPresContext* pc = scrollFrame->PresContext();
+    if (Element* root = pc->Document()->GetRootElement()) {
+      if (nsIFrame* frameOfRoot = root->GetPrimaryFrame()) {
+        return frameOfRoot->Style();
+      }
+      // If the root doesn't have primary frame, get the computed style
+      // from the element. Dropping the strong reference is fine because
+      // the style should be held strongly by the element.
+      RefPtr<ComputedStyle> style = pc->StyleSet()->ResolveServoStyle(root);
+      return style.get();
+    }
+  }
+  return scrollFrame->Style();
+}

layout/base/nsLayoutUtils.h

@@ -3111,6 +3111,15 @@ public:
     return ResolveToLength<true>(aGap, aPercentageBasis);
   }
 
+  /**
+   * Get the computed style for the given scrollbar part frame.
+   *
+   * Generally it returns the computed style of the scroll frame which
+   * contains the given part frame. If the scroll frame is for the
+   * viewport, the computed style of the document element is returned.
+   */
+  static ComputedStyle* StyleForScrollbar(nsIFrame* aScrollbarPart);
+
 private:
   static uint32_t sFontSizeInflationEmPerLine;
   static uint32_t sFontSizeInflationMinTwips;

widget/windows/nsNativeThemeWin.cpp

@@ -19,6 +19,7 @@
 #include "nsIContent.h"
 #include "nsIContentInlines.h"
 #include "nsIFrame.h"
+#include "nsLayoutUtils.h"
 #include "nsNameSpaceManager.h"
 #include "nsLookAndFeel.h"
 #include "nsMenuFrame.h"
@@ -1551,10 +1552,12 @@ nsNativeThemeWin::DrawWidgetBackground(gfxContext* aContext,
                                        const nsRect& aRect,
                                        const nsRect& aDirtyRect)
 {
-  if (aFrame->StyleUserInterface()->HasCustomScrollbars() &&
-      IsWidgetScrollbarPart(aWidgetType)) {
-    return DrawCustomScrollbarPart(aContext, aFrame, aWidgetType,
-                                   aRect, aDirtyRect);
+  if (IsWidgetScrollbarPart(aWidgetType)) {
+    ComputedStyle* style = nsLayoutUtils::StyleForScrollbar(aFrame);
+    if (style->StyleUserInterface()->HasCustomScrollbars()) {
+      return DrawCustomScrollbarPart(aContext, aFrame, style,
+                                     aWidgetType, aRect, aDirtyRect);
+    }
   }
 
   HANDLE theme = GetTheme(aWidgetType);
@@ -4263,19 +4266,19 @@ GetScrollbarArrowColor(nscolor aTrackColor)
 nsresult
 nsNativeThemeWin::DrawCustomScrollbarPart(gfxContext* aContext,
                                           nsIFrame* aFrame,
+                                          ComputedStyle* aStyle,
                                           uint8_t aWidgetType,
                                           const nsRect& aRect,
                                           const nsRect& aClipRect)
 {
-  ComputedStyle* style = aFrame->Style();
-  MOZ_ASSERT(!style->StyleUserInterface()->mScrollbarFaceColor.mIsAuto ||
-             !style->StyleUserInterface()->mScrollbarTrackColor.mIsAuto);
+  MOZ_ASSERT(!aStyle->StyleUserInterface()->mScrollbarFaceColor.mIsAuto ||
+             !aStyle->StyleUserInterface()->mScrollbarTrackColor.mIsAuto);
 
   gfxRect tr(aRect.X(), aRect.Y(), aRect.Width(), aRect.Height()),
           dr(aClipRect.X(), aClipRect.Y(),
              aClipRect.Width(), aClipRect.Height());
 
-  nscolor trackColor = GetScrollbarTrackColor(style);
+  nscolor trackColor = GetScrollbarTrackColor(aStyle);
   HBRUSH dcBrush = (HBRUSH) GetStockObject(DC_BRUSH);
 
   gfxFloat p2a = gfxFloat(aFrame->PresContext()->AppUnitsPerDevPixel());
@@ -4311,7 +4314,7 @@ nsNativeThemeWin::DrawCustomScrollbarPart(gfxContext* aContext,
           tr2.Deflate(0, dev2css);
         }
         nativeDrawing.TransformToNativeRect(tr2, widgetRect);
-        nscolor faceColor = GetScrollbarFaceColor(style);
+        nscolor faceColor = GetScrollbarFaceColor(aStyle);
         ::SetDCBrushColor(hdc, ToColorRef(faceColor));
         ::FillRect(hdc, &widgetRect, dcBrush);
         break;

widget/windows/nsNativeThemeWin.h

@@ -116,6 +116,7 @@ protected:
                        HBRUSH defaultBack);
   nsresult DrawCustomScrollbarPart(gfxContext* aContext,
                                    nsIFrame* aFrame,
+                                   mozilla::ComputedStyle* aStyle,
                                    uint8_t aWidgetType,
                                    const nsRect& aRect,
                                    const nsRect& aClipRect);