Issue #2765 - Part 1: Scrollbar width should be treated as a non-inherited property

Note: non-inherited properties are stored in "reset" style structs.

Previous implementation treats it as an inherited property, which
doesn't match the spec.

This also fixes the incorrect behavior when using the `unset` value
for this property by specifying SETVAL_UNSET_INITIAL in the mask.

layout/generic/nsGfxScrollFrame.cpp

@@ -1058,7 +1058,7 @@ nsHTMLScrollFrame::Reflow(nsPresContext* aPresContext,
     // This is only needed for root element because scrollbars of non-
     // root elements with "scrollbar-width: none" is already suppressed
     // in ScrollFrameHelper::CreateAnonymousContent.
-    if (this->StyleUserInterface()->mScrollbarWidth == StyleScrollbarWidth::None) {
+    if (this->StyleUIReset()->mScrollbarWidth == StyleScrollbarWidth::None) {
       state.mVScrollbar = ShowScrollbar::Never;
       state.mHScrollbar = ShowScrollbar::Never;
     }
@@ -4415,7 +4415,7 @@ ScrollFrameHelper::CreateAnonymousContent(
     canHaveHorizontal = true;
     canHaveVertical = true;
   } else {
-    if (mOuter->StyleUserInterface()->mScrollbarWidth == StyleScrollbarWidth::None) {
+    if (mOuter->StyleUIReset()->mScrollbarWidth == StyleScrollbarWidth::None) {
       // If scrollbar-width is none, don't generate scrollbars.
       canHaveHorizontal = false;
       canHaveVertical = false;
@@ -4448,7 +4448,7 @@ ScrollFrameHelper::CreateAnonymousContent(
                                           kNameSpaceID_XUL,
                                           nsIDOMNode::ELEMENT_NODE);
   NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
-  StyleScrollbarWidth scrollWidth = mOuter->StyleUserInterface()->mScrollbarWidth;
+  StyleScrollbarWidth scrollWidth = mOuter->StyleUIReset()->mScrollbarWidth;
 
   if (canHaveHorizontal) {
     RefPtr<NodeInfo> ni = nodeInfo;

layout/style/nsCSSPropList.h

@@ -3723,7 +3723,7 @@ CSS_PROP_DISPLAY(
     kScrollSnapTypeKTable,
     CSS_PROP_NO_OFFSET,
     eStyleAnimType_Discrete)
-CSS_PROP_USERINTERFACE(
+CSS_PROP_UIRESET(
     scrollbar-width,
     scrollbar_width,
     ScrollbarWidth,

layout/style/nsComputedDOMStyle.cpp

@@ -3302,7 +3302,7 @@ nsComputedDOMStyle::DoGetScrollbarWidth()
 {
   RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
   val->SetIdent(
-    nsCSSProps::ValueToKeywordEnum(StyleUserInterface()->mScrollbarWidth,
+    nsCSSProps::ValueToKeywordEnum(StyleUIReset()->mScrollbarWidth,
                                    nsCSSProps::kScrollbarWidthKTable));
   return val.forget();
 }

layout/style/nsRuleNode.cpp

@@ -5378,14 +5378,6 @@ nsRuleNode::ComputeUserInterfaceData(void* aStartStruct,
   setComplexColor(aRuleData->ValueForCaretColor(),
                   &nsStyleUserInterface::mCaretColor);
 
-  // scrollbar-width: auto, thin, none
-  SetValue(*aRuleData->ValueForScrollbarWidth(),
-           ui->mScrollbarWidth,
-           conditions,
-           SETVAL_ENUMERATED,
-           parentUI->mScrollbarWidth,
-           StyleScrollbarWidth::Auto);
-
   COMPUTE_END_INHERITED(UserInterface, ui)
 }
 
@@ -5434,6 +5426,14 @@ nsRuleNode::ComputeUIResetData(void* aStartStruct,
            parentUI->mWindowShadow,
            NS_STYLE_WINDOW_SHADOW_DEFAULT);
 
+  // scrollbar-width: auto, thin, none
+  SetValue(*aRuleData->ValueForScrollbarWidth(),
+           ui->mScrollbarWidth,
+           conditions,
+           SETVAL_ENUMERATED | SETVAL_UNSET_INITIAL,
+           parentUI->mScrollbarWidth,
+           StyleScrollbarWidth::Auto);
+
   COMPUTE_END_RESET(UIReset, ui)
 }
 

layout/style/nsStyleStruct.cpp

@@ -3924,7 +3924,6 @@ nsStyleUserInterface::nsStyleUserInterface(nsPresContext* aContext)
   , mPointerEvents(NS_STYLE_POINTER_EVENTS_AUTO)
   , mCursor(NS_STYLE_CURSOR_AUTO)
   , mCaretColor(StyleComplexColor::Auto())
-  , mScrollbarWidth(StyleScrollbarWidth::Auto)
 {
   MOZ_COUNT_CTOR(nsStyleUserInterface);
 }
@@ -3937,7 +3936,6 @@ nsStyleUserInterface::nsStyleUserInterface(const nsStyleUserInterface& aSource)
   , mCursor(aSource.mCursor)
   , mCursorImages(aSource.mCursorImages)
   , mCaretColor(aSource.mCaretColor)
-  , mScrollbarWidth(aSource.mScrollbarWidth)
 {
   MOZ_COUNT_CTOR(nsStyleUserInterface);
 }
@@ -3990,13 +3988,6 @@ nsStyleUserInterface::CalcDifference(const nsStyleUserInterface& aNewData) const
     hint |= nsChangeHint_RepaintFrame;
   }
 
-  if (mScrollbarWidth != aNewData.mScrollbarWidth) {
-    // For scrollbar-width change, we need some special handling similar
-    // to overflow properties. Specifically, we may need to reconstruct
-    // the scrollbar or force reflow of the viewport scrollbar.
-    hint |= nsChangeHint_ScrollbarChange;
-  }
-
   return hint;
 }
 
@@ -4010,6 +4001,7 @@ nsStyleUIReset::nsStyleUIReset(nsPresContext* aContext)
   , mIMEMode(NS_STYLE_IME_MODE_AUTO)
   , mWindowDragging(StyleWindowDragging::Default)
   , mWindowShadow(NS_STYLE_WINDOW_SHADOW_DEFAULT)
+  , mScrollbarWidth(StyleScrollbarWidth::Auto)
 {
   MOZ_COUNT_CTOR(nsStyleUIReset);
 }
@@ -4020,6 +4012,7 @@ nsStyleUIReset::nsStyleUIReset(const nsStyleUIReset& aSource)
   , mIMEMode(aSource.mIMEMode)
   , mWindowDragging(aSource.mWindowDragging)
   , mWindowShadow(aSource.mWindowShadow)
+  , mScrollbarWidth(aSource.mScrollbarWidth)
 {
   MOZ_COUNT_CTOR(nsStyleUIReset);
 }
@@ -4050,6 +4043,13 @@ nsStyleUIReset::CalcDifference(const nsStyleUIReset& aNewData) const
     return nsChangeHint_SchedulePaint;
   }
 
+  if (mScrollbarWidth != aNewData.mScrollbarWidth) {
+    // For scrollbar-width change, we need some special handling similar
+    // to overflow properties. Specifically, we may need to reconstruct
+    // the scrollbar or force reflow of the viewport scrollbar.
+    return nsChangeHint_ScrollbarChange;
+  }
+
   return nsChangeHint(0);
 }
 

layout/style/nsStyleStruct.h

@@ -3267,6 +3267,7 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleUIReset
   uint8_t                      mIMEMode;        // [reset]
   mozilla::StyleWindowDragging mWindowDragging; // [reset]
   uint8_t                      mWindowShadow;   // [reset]
+  mozilla::StyleScrollbarWidth mScrollbarWidth; // [reset]
 };
 
 struct nsCursorImage
@@ -3344,7 +3345,6 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleUserInterface
   uint8_t mCursor;                            // [inherited] See nsStyleConsts.h
   nsTArray<nsCursorImage> mCursorImages;      // [inherited] images and coords
   mozilla::StyleComplexColor mCaretColor;     // [inherited]
-  mozilla::StyleScrollbarWidth mScrollbarWidth;
 
   inline uint8_t GetEffectivePointerEvents(nsIFrame* aFrame) const;
 };

widget/windows/nsNativeThemeWin.cpp

@@ -1572,7 +1572,7 @@ GetThemeDpiScaleFactor(nsIFrame* aFrame)
 static bool
 IsScrollbarWidthThin(nsIFrame* aFrame)
 {
-  return aFrame->StyleUserInterface()->mScrollbarWidth == StyleScrollbarWidth::Thin;
+  return aFrame->StyleUIReset()->mScrollbarWidth == StyleScrollbarWidth::Thin;
 }
 
 NS_IMETHODIMP