Bug 1554571 - Part 3: Cache computed styles of scrollbar part anonymous content. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D33123

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

browser/base/content/test/static/browser_parsable_css.js

@@ -30,6 +30,9 @@ let whitelist = [
   {sourceName: /\b(html|mathml|ua)\.css$/i,
    errorMessage: /Unknown property.*-moz-/i,
    isFromDevTools: false},
+  {sourceName: /minimal-xul\.css$/i,
+   errorMessage: /Unknown property.*-moz-list-reversed/i,
+   isFromDevTools: false},
   // Reserved to UA sheets unless layout.css.overflow-clip-box.enabled flipped to true.
   {sourceName: /(?:res|gre-resources)\/forms\.css$/i,
    errorMessage: /Unknown property.*overflow-clip-box/i,

layout/base/nsCSSFrameConstructor.cpp

@@ -3843,6 +3843,18 @@ static void SetFlagsOnSubtree(nsIContent* aNode, uintptr_t aFlagsToSet) {
   }
 }
 
+static void GatherSubtreeElements(Element* aElement,
+                                  nsTArray<Element*>& aElements) {
+  aElements.AppendElement(aElement);
+  StyleChildrenIterator iter(aElement);
+  for (nsIContent* c = iter.GetNextChild(); c; c = iter.GetNextChild()) {
+    if (!c->IsElement()) {
+      continue;
+    }
+    GatherSubtreeElements(c->AsElement(), aElements);
+  }
+}
+
 nsresult nsCSSFrameConstructor::GetAnonymousContent(
     nsIContent* aParent, nsIFrame* aParentFrame,
     nsTArray<nsIAnonymousContentCreator::ContentInfo>& aContent) {
@@ -3883,11 +3895,74 @@ nsresult nsCSSFrameConstructor::GetAnonymousContent(
     }
   }
 
+  // Some situations where we don't cache anonymous content styles:
+  //
+  // * when visibility is anything other than visible; we rely on visibility
+  //   inheriting into anonymous content, but don't bother adding this state
+  //   to the AnonymousContentKey, since it's not so common
+  //
+  // * when the medium is anything other than screen; some UA style sheet rules
+  //   apply in e.g. print medium, and will give different results from the
+  //   cached styles
+  bool allowStyleCaching =
+      StaticPrefs::layout_css_cached_scrollbar_styles_enabled() &&
+      aParentFrame->StyleVisibility()->mVisible == NS_STYLE_VISIBILITY_VISIBLE &&
+      mPresShell->GetPresContext()->Medium() == nsGkAtoms::screen;
+
+  // Compute styles for the anonymous content tree.
   ServoStyleSet* styleSet = mPresShell->StyleSet();
-  // Eagerly compute styles for the anonymous content tree.
   for (auto& info : aContent) {
-    if (info.mContent->IsElement()) {
-      styleSet->StyleNewSubtree(info.mContent->AsElement());
+    Element* e = Element::FromNode(info.mContent);
+    if (!e) {
+      continue;
+    }
+
+    if (info.mKey == AnonymousContentKey::None || !allowStyleCaching) {
+      // Most NAC subtrees do not use caching of computed styles.  Just go
+      // ahead and eagerly style the subtree.
+      styleSet->StyleNewSubtree(e);
+      continue;
+    }
+
+    // We have a NAC subtree for which we can use cached styles.
+    AutoTArray<RefPtr<ComputedStyle>, 2> cachedStyles;
+    AutoTArray<Element*, 2> elements;
+
+    GatherSubtreeElements(e, elements);
+    styleSet->GetCachedAnonymousContentStyles(info.mKey, cachedStyles);
+
+    if (cachedStyles.IsEmpty()) {
+      // We haven't store cached styles for this kind of NAC subtree yet.
+      // Eagerly compute those styles, then cache them for later.
+      styleSet->StyleNewSubtree(e);
+      for (Element* e : elements) {
+        if (e->HasServoData()) {
+          cachedStyles.AppendElement(ServoStyleSet::ResolveServoStyle(*e));
+        } else {
+          cachedStyles.AppendElement(nullptr);
+        }
+      }
+      styleSet->PutCachedAnonymousContentStyles(info.mKey,
+                                                std::move(cachedStyles));
+      continue;
+    }
+
+    // We previously stored cached styles for this kind of NAC subtree.
+    // Iterate over them and set them on the subtree's elements.
+    MOZ_ASSERT(cachedStyles.Length() == elements.Length(),
+               "should always produce the same size NAC subtree");
+    for (size_t i = 0, len = cachedStyles.Length(); i != len; ++i) {
+      if (cachedStyles[i]) {
+#ifdef DEBUG
+        // Assert that our cached style is the same as one we could compute.
+        RefPtr<ComputedStyle> cs = styleSet->ResolveStyleLazily(*elements[i]);
+        MOZ_ASSERT(
+            cachedStyles[i]->EqualForCachedAnonymousContentStyle(*cs),
+            "cached anonymous content styles should be identical to those we "
+            "would compute normally");
+#endif
+        Servo_SetExplicitStyle(elements[i], cachedStyles[i]);
+      }
     }
   }
 

layout/generic/AnonymousContentKey.h

@@ -0,0 +1,56 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* values to identify particular subtrees of native anonymous content */
+
+#ifndef mozilla_AnonymousContentKey_h
+#define mozilla_AnonymousContentKey_h
+
+namespace mozilla {
+
+// clang-format off
+
+// We currently use cached anonymous content styles only for scrollbar parts,
+// and we can fit the type of scrollbar part element along with its different
+// options (such as orientation, and other attribute values that can affect
+// styling) into a uint8_t.
+//
+// The lower three bits hold a Type_* value identifying the type of
+// element, and the remaining bits store Flag_* values.
+//
+// A value of 0 is used to represent an anonymous content subtree that we don't
+// cache styles for.
+enum class AnonymousContentKey : uint8_t {
+  None                           = 0x00,
+
+  // all
+  Type_ScrollCorner              = 0x01,
+  Type_Resizer                   = 0x02,
+  Type_Scrollbar                 = 0x03,
+  Type_ScrollbarButton           = 0x04,
+  Type_Slider                    = 0x05,
+
+  // scrollbar, scrollbarbutton, slider
+  Flag_Vertical                  = 0x08,
+
+  // resizer
+  Flag_Resizer_Right             = 0x08,
+  Flag_Resizer_Bottom            = 0x10,
+  Flag_Resizer_Flip              = 0x20,
+
+  // scrollbarbutton
+  Flag_ScrollbarButton_Down      = 0x10,
+  Flag_ScrollbarButton_Bottom    = 0x20,
+  Flag_ScrollbarButton_Decrement = 0x40,
+};
+
+// clang-format on
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(AnonymousContentKey)
+
+}  // namespace mozilla
+
+#endif  // mozilla_AnonymousContentKey_h

layout/generic/moz.build

@@ -134,6 +134,7 @@ EXPORTS += [
 EXPORTS.mozilla += [
     '!FrameIdList.h',
     '!FrameTypeList.h',
+    'AnonymousContentKey.h',
     'AspectRatio.h',
     'AutoCopyListener.h',
     'ColumnUtils.h',

layout/generic/nsGfxScrollFrame.cpp

@@ -4756,6 +4756,51 @@ void ScrollFrameHelper::ReloadChildFrames() {
   }
 }
 
+already_AddRefed<Element> ScrollFrameHelper::MakeScrollbar(
+    NodeInfo* aNodeInfo, bool aVertical, AnonymousContentKey& aKey) {
+  MOZ_ASSERT(aNodeInfo);
+  MOZ_ASSERT(
+      aNodeInfo->Equals(nsGkAtoms::scrollbar, nullptr, kNameSpaceID_XUL));
+
+  static constexpr nsLiteralString kOrientValues[2] = {
+      NS_LITERAL_STRING("horizontal"),
+      NS_LITERAL_STRING("vertical"),
+  };
+
+  aKey = AnonymousContentKey::Type_Scrollbar;
+  if (aVertical) {
+    aKey |= AnonymousContentKey::Flag_Vertical;
+  }
+
+  RefPtr<Element> e;
+  NS_TrustedNewXULElement(getter_AddRefs(e), do_AddRef(aNodeInfo));
+
+#ifdef DEBUG
+  // Scrollbars can get restyled by theme changes.  Whether such a restyle
+  // will actually reconstruct them correctly if it involves a frame
+  // reconstruct... I don't know.  :(
+  e->SetProperty(nsGkAtoms::restylableAnonymousNode,
+                 reinterpret_cast<void*>(true));
+#endif  // DEBUG
+
+  e->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, kOrientValues[aVertical],
+             false);
+  e->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
+             NS_LITERAL_STRING("always"), false);
+
+  if (mIsRoot) {
+    e->SetProperty(nsGkAtoms::docLevelNativeAnonymousContent,
+                   reinterpret_cast<void*>(true));
+    e->SetAttr(kNameSpaceID_None, nsGkAtoms::root_, NS_LITERAL_STRING("true"),
+               false);
+
+    // Don't bother making style caching take [root="true"] styles into account.
+    aKey = AnonymousContentKey::None;
+  }
+
+  return e.forget();
+}
+
 bool ScrollFrameHelper::IsForTextControlWithNoScrollbars() const {
   nsIFrame* parent = mOuter->GetParent();
   // The anonymous <div> used by <inputs> never gets scrollbars.
@@ -4773,6 +4818,8 @@ bool ScrollFrameHelper::IsForTextControlWithNoScrollbars() const {
 
 nsresult ScrollFrameHelper::CreateAnonymousContent(
     nsTArray<nsIAnonymousContentCreator::ContentInfo>& aElements) {
+  typedef nsIAnonymousContentCreator::ContentInfo ContentInfo;
+
   nsPresContext* presContext = mOuter->PresContext();
 
   // Don't create scrollbars if we're an SVG document being used as an image,
@@ -4833,62 +4880,28 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
 
   nsNodeInfoManager* nodeInfoManager =
       presContext->Document()->NodeInfoManager();
-  RefPtr<NodeInfo> nodeInfo = nodeInfoManager->GetNodeInfo(
-      nsGkAtoms::scrollbar, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
-  NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
 
-  if (canHaveHorizontal) {
-    RefPtr<NodeInfo> ni = nodeInfo;
-    NS_TrustedNewXULElement(getter_AddRefs(mHScrollbarContent), ni.forget());
-#ifdef DEBUG
-    // Scrollbars can get restyled by theme changes.  Whether such a restyle
-    // will actually reconstruct them correctly if it involves a frame
-    // reconstruct... I don't know.  :(
-    mHScrollbarContent->SetProperty(nsGkAtoms::restylableAnonymousNode,
-                                    reinterpret_cast<void*>(true));
-#endif  // DEBUG
+  {
+    RefPtr<NodeInfo> nodeInfo = nodeInfoManager->GetNodeInfo(
+        nsGkAtoms::scrollbar, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
+    NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
 
-    mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::orient,
-                                NS_LITERAL_STRING("horizontal"), false);
-    mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
-                                NS_LITERAL_STRING("always"), false);
-    if (mIsRoot) {
-      mHScrollbarContent->SetProperty(nsGkAtoms::docLevelNativeAnonymousContent,
-                                      reinterpret_cast<void*>(true));
-
-      mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::root_,
-                                  NS_LITERAL_STRING("true"), false);
+    if (canHaveHorizontal) {
+      AnonymousContentKey key;
+      mHScrollbarContent = MakeScrollbar(nodeInfo, /* aVertical */ false, key);
+      aElements.AppendElement(ContentInfo(mHScrollbarContent, key));
     }
-    if (!aElements.AppendElement(mHScrollbarContent))
-      return NS_ERROR_OUT_OF_MEMORY;
-  }
 
-  if (canHaveVertical) {
-    RefPtr<NodeInfo> ni = nodeInfo;
-    NS_TrustedNewXULElement(getter_AddRefs(mVScrollbarContent), ni.forget());
-#ifdef DEBUG
-    // Scrollbars can get restyled by theme changes.  Whether such a restyle
-    // will actually reconstruct them correctly if it involves a frame
-    // reconstruct... I don't know.  :(
-    mVScrollbarContent->SetProperty(nsGkAtoms::restylableAnonymousNode,
-                                    reinterpret_cast<void*>(true));
-#endif  // DEBUG
-
-    mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::orient,
-                                NS_LITERAL_STRING("vertical"), false);
-    mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
-                                NS_LITERAL_STRING("always"), false);
-    if (mIsRoot) {
-      mVScrollbarContent->SetProperty(nsGkAtoms::docLevelNativeAnonymousContent,
-                                      reinterpret_cast<void*>(true));
-      mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::root_,
-                                  NS_LITERAL_STRING("true"), false);
+    if (canHaveVertical) {
+      AnonymousContentKey key;
+      mVScrollbarContent = MakeScrollbar(nodeInfo, /* aVertical */ true, key);
+      aElements.AppendElement(ContentInfo(mVScrollbarContent, key));
     }
-    if (!aElements.AppendElement(mVScrollbarContent))
-      return NS_ERROR_OUT_OF_MEMORY;
   }
 
   if (isResizable) {
+    AnonymousContentKey key = AnonymousContentKey::Type_Resizer;
+
     RefPtr<NodeInfo> nodeInfo;
     nodeInfo = nodeInfoManager->GetNodeInfo(
         nsGkAtoms::resizer, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
@@ -4901,20 +4914,25 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
       case StyleResize::Horizontal:
         if (IsScrollbarOnRight()) {
           dir.AssignLiteral("right");
+          key |= AnonymousContentKey::Flag_Resizer_Right;
         } else {
           dir.AssignLiteral("left");
         }
         break;
       case StyleResize::Vertical:
         dir.AssignLiteral("bottom");
+        key |= AnonymousContentKey::Flag_Resizer_Bottom;
         if (!IsScrollbarOnRight()) {
           mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::flip,
                                    EmptyString(), false);
+          key |= AnonymousContentKey::Flag_Resizer_Flip;
         }
         break;
       case StyleResize::Both:
+        key |= AnonymousContentKey::Flag_Resizer_Bottom;
         if (IsScrollbarOnRight()) {
           dir.AssignLiteral("bottomright");
+          key |= AnonymousContentKey::Flag_Resizer_Right;
         } else {
           dir.AssignLiteral("bottomleft");
         }
@@ -4940,12 +4958,13 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
     mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
                              NS_LITERAL_STRING("always"), false);
 
-    if (!aElements.AppendElement(mResizerContent))
-      return NS_ERROR_OUT_OF_MEMORY;
+    aElements.AppendElement(ContentInfo(mResizerContent, key));
   }
 
   if (canHaveHorizontal && canHaveVertical) {
-    nodeInfo =
+    AnonymousContentKey key = AnonymousContentKey::Type_ScrollCorner;
+
+    RefPtr<NodeInfo> nodeInfo =
         nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollcorner, nullptr,
                                      kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
     NS_TrustedNewXULElement(getter_AddRefs(mScrollCornerContent),
@@ -4955,8 +4974,15 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
           nsGkAtoms::docLevelNativeAnonymousContent,
           reinterpret_cast<void*>(true));
     }
-    if (!aElements.AppendElement(mScrollCornerContent))
-      return NS_ERROR_OUT_OF_MEMORY;
+    aElements.AppendElement(ContentInfo(mScrollCornerContent, key));
+  }
+
+  // Don't cache styles if we are a child of a <select> element, since we have
+  // some UA style sheet rules that depend on the <select>'s attributes.
+  if (mOuter->GetContent()->IsHTMLElement(nsGkAtoms::select)) {
+    for (auto& info : aElements) {
+      info.mKey = AnonymousContentKey::None;
+    }
   }
 
   return NS_OK;

layout/generic/nsGfxScrollFrame.h

@@ -513,12 +513,17 @@ class ScrollFrameHelper : public nsIReflowCallback {
   void UpdateMinimumScaleSize(const nsRect& aScrollableOverflow,
                               const nsSize& aICBSize);
 
+
   // Return the scroll frame's "true outer size".
   // This is mOuter->GetSize(), except when mOuter has been sized to reflect
   // a virtual (layout) viewport in which case this returns the outer size
   // used to size the physical (visual) viewport.
   nsSize TrueOuterSize() const;
 
+  already_AddRefed<Element> MakeScrollbar(dom::NodeInfo* aNodeInfo,
+                                          bool aVertical,
+                                          AnonymousContentKey& aKey);
+
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<mozilla::dom::Element> mHScrollbarContent;
   nsCOMPtr<mozilla::dom::Element> mVScrollbarContent;

layout/generic/nsIAnonymousContentCreator.h

@@ -12,6 +12,7 @@
 #ifndef nsIAnonymousContentCreator_h___
 #define nsIAnonymousContentCreator_h___
 
+#include "mozilla/AnonymousContentKey.h"
 #include "mozilla/ComputedStyle.h"
 
 #include "nsQueryFrame.h"
@@ -31,9 +32,12 @@ class nsIAnonymousContentCreator {
   NS_DECL_QUERYFRAME_TARGET(nsIAnonymousContentCreator)
 
   struct ContentInfo {
-    explicit ContentInfo(nsIContent* aContent) : mContent(aContent) {}
+    explicit ContentInfo(nsIContent* aContent,
+                         mozilla::AnonymousContentKey aKey = mozilla::AnonymousContentKey::None)
+        : mContent(aContent), mKey(aKey) {}
 
     nsIContent* mContent;
+    mozilla::AnonymousContentKey mKey;
   };
 
   /**

layout/style/ComputedStyle.cpp

@@ -394,4 +394,17 @@ void ComputedStyle::AddSizeOfIncludingThis(nsWindowSizes& aSizes,
   mCachedInheritingStyles.AddSizeOfIncludingThis(aSizes, aCVsSize);
 }
 
+#ifdef DEBUG
+bool ComputedStyle::EqualForCachedAnonymousContentStyle(
+    const ComputedStyle& aOther) const {
+  // One thing we can't add UA rules to prevent is different -x-lang
+  // values being inherited in.  So we use this FFI function function rather
+  // than rely on CalcStyleDifference, which can't tell us which specific
+  // properties have changed.
+  return Servo_ComputedValues_EqualForCachedAnonymousContentStyle(this,
+                                                                  &aOther);
+}
+
+#endif
+
 }  // namespace mozilla

layout/style/ComputedStyle.h

@@ -240,6 +240,10 @@ class ComputedStyle {
   nsChangeHint CalcStyleDifference(const ComputedStyle& aNewContext,
                                    uint32_t* aEqualStructs) const;
 
+#ifdef DEBUG
+  bool EqualForCachedAnonymousContentStyle(const ComputedStyle&) const;
+#endif
+
  public:
   /**
    * Get a color that depends on link-visitedness using this and

layout/style/GeckoBindings.cpp

@@ -2037,3 +2037,7 @@ void Gecko_LoadData_DeregisterLoad(const StyleLoadData* aData) {
   MOZ_ASSERT(aData->load_id != 0);
   ImageLoader::DeregisterCSSImageFromAllLoaders(*aData);
 }
+
+void Gecko_PrintfStderr(const nsCString* aStr) {
+  printf_stderr("%s", aStr->get());
+}

layout/style/GeckoBindings.h

@@ -732,6 +732,8 @@ bool Gecko_MediaFeatures_IsResourceDocument(const mozilla::dom::Document*);
 nsAtom* Gecko_MediaFeatures_GetOperatingSystemVersion(
     const mozilla::dom::Document*);
 
+void Gecko_PrintfStderr(const nsCString*);
+
 }  // extern "C"
 
 #endif  // mozilla_GeckoBindings_h

layout/style/ServoStyleSet.cpp

@@ -990,6 +990,8 @@ bool ServoStyleSet::EnsureUniqueInnerOnCSSSheets() {
 void ServoStyleSet::ClearCachedStyleData() {
   ClearNonInheritingComputedStyles();
   Servo_StyleSet_RebuildCachedData(mRawSet.get());
+  mCachedAnonymousContentStyles.Clear();
+  PodArrayZero(mCachedAnonymousContentStyleIndexes);
 }
 
 void ServoStyleSet::ForceDirtyAllShadowStyles() {

layout/style/ServoStyleSet.h

@@ -7,6 +7,7 @@
 #ifndef mozilla_ServoStyleSet_h
 #define mozilla_ServoStyleSet_h
 
+#include "mozilla/AnonymousContentKey.h"
 #include "mozilla/AtomArray.h"
 #include "mozilla/EnumeratedArray.h"
 #include "mozilla/EventStates.h"
@@ -294,8 +295,8 @@ class ServoStyleSet {
    * Clears any cached style data that may depend on all sorts of computed
    * values.
    *
-   * Right now this clears the non-inheriting ComputedStyle cache, and resets
-   * the default computed values.
+   * Right now this clears the non-inheriting ComputedStyle cache, resets the
+   * default computed values, and clears cached anonymous content style.
    *
    * This does _not_, however, clear the stylist.
    */
@@ -553,6 +554,42 @@ class ServoStyleSet {
                   nsCSSAnonBoxes::NonInheriting::_Count, RefPtr<ComputedStyle>>
       mNonInheritingComputedStyles;
 
+ public:
+  void PutCachedAnonymousContentStyles(
+      AnonymousContentKey aKey, nsTArray<RefPtr<ComputedStyle>>&& aStyles) {
+    auto index = static_cast<size_t>(aKey);
+
+    MOZ_ASSERT(mCachedAnonymousContentStyles.Length() + aStyles.Length() < 256,
+               "(index, length) pairs must be bigger");
+    MOZ_ASSERT(mCachedAnonymousContentStyleIndexes[index].second == 0,
+               "shouldn't need to overwrite existing cached styles");
+    MOZ_ASSERT(!aStyles.IsEmpty(), "should have some styles to cache");
+
+    mCachedAnonymousContentStyleIndexes[index] = std::make_pair(
+        mCachedAnonymousContentStyles.Length(), aStyles.Length());
+    mCachedAnonymousContentStyles.AppendElements(std::move(aStyles));
+  }
+
+  void GetCachedAnonymousContentStyles(
+      AnonymousContentKey aKey, nsTArray<RefPtr<ComputedStyle>>& aStyles) {
+    auto index = static_cast<size_t>(aKey);
+    auto loc = mCachedAnonymousContentStyleIndexes[index];
+    aStyles.AppendElements(mCachedAnonymousContentStyles.Elements() + loc.first,
+                           loc.second);
+  }
+
+ private:
+  // Map of AnonymousContentKey values to an (index, length) pair pointing into
+  // mCachedAnonymousContentStyles.
+  //
+  // We assert that the index and length values fit into uint8_ts.
+  std::pair<uint8_t, uint8_t>
+      mCachedAnonymousContentStyleIndexes[1
+                                          << sizeof(AnonymousContentKey) * 8]{};
+
+  // Stores cached ComputedStyles for certain native anonymous content.
+  nsTArray<RefPtr<ComputedStyle>> mCachedAnonymousContentStyles;
+
   StylistState mStylistState = StylistState::NotDirty;
   bool mAuthorStyleDisabled = false;
   bool mNeedsRestyleAfterEnsureUniqueInner = false;

layout/xul/nsScrollbarFrame.cpp

@@ -282,6 +282,48 @@ int32_t nsScrollbarFrame::MoveToNewPosition() {
   return curpos;
 }
 
+static already_AddRefed<Element> MakeScrollbarButton(
+    dom::NodeInfo* aNodeInfo, bool aVertical, bool aBottom, bool aDown,
+    AnonymousContentKey& aKey) {
+  MOZ_ASSERT(aNodeInfo);
+  MOZ_ASSERT(
+      aNodeInfo->Equals(nsGkAtoms::scrollbarbutton, nullptr, kNameSpaceID_XUL));
+
+  static constexpr nsLiteralString kSbattrValues[2][2] = {
+      {
+          NS_LITERAL_STRING("scrollbar-up-top"),
+          NS_LITERAL_STRING("scrollbar-up-bottom"),
+      },
+      {
+          NS_LITERAL_STRING("scrollbar-down-top"),
+          NS_LITERAL_STRING("scrollbar-down-bottom"),
+      },
+  };
+
+  static constexpr nsLiteralString kTypeValues[2] = {
+      NS_LITERAL_STRING("decrement"),
+      NS_LITERAL_STRING("increment"),
+  };
+
+  aKey = AnonymousContentKey::Type_ScrollbarButton;
+  if (aVertical) {
+    aKey |= AnonymousContentKey::Flag_Vertical;
+  }
+  if (aBottom) {
+    aKey |= AnonymousContentKey::Flag_ScrollbarButton_Bottom;
+  }
+  if (aDown) {
+    aKey |= AnonymousContentKey::Flag_ScrollbarButton_Down;
+  }
+
+  RefPtr<Element> e;
+  NS_TrustedNewXULElement(getter_AddRefs(e), do_AddRef(aNodeInfo));
+  e->SetAttr(kNameSpaceID_None, nsGkAtoms::sbattr,
+             kSbattrValues[aDown][aBottom], false);
+  e->SetAttr(kNameSpaceID_None, nsGkAtoms::type, kTypeValues[aDown], false);
+  return e.forget();
+}
+
 nsresult nsScrollbarFrame::CreateAnonymousContent(
     nsTArray<ContentInfo>& aElements) {
   nsNodeInfoManager* nodeInfoManager = mContent->NodeInfo()->NodeInfoManager();
@@ -296,80 +338,79 @@ nsresult nsScrollbarFrame::CreateAnonymousContent(
 
   nsAutoString orient;
   el->GetAttr(kNameSpaceID_None, nsGkAtoms::orient, orient);
+  bool vertical = orient.EqualsLiteral("vertical");
 
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mUpTopButton),
+  RefPtr<dom::NodeInfo> sbbNodeInfo =
       nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbarbutton, nullptr,
-                                   kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
-  mUpTopButton->SetAttr(kNameSpaceID_None, nsGkAtoms::sbattr,
-                        NS_LITERAL_STRING("scrollbar-up-top"), false);
-  mUpTopButton->SetAttr(kNameSpaceID_None, nsGkAtoms::type,
-                        NS_LITERAL_STRING("decrement"), false);
+                                   kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
 
-  if (!aElements.AppendElement(mUpTopButton)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+  {
+    AnonymousContentKey key;
+    mUpTopButton =
+        MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ false,
+                            /* aDown */ false, key);
+    aElements.AppendElement(ContentInfo(mUpTopButton, key));
   }
 
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mDownTopButton),
-      nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbarbutton, nullptr,
-                                   kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
-  mDownTopButton->SetAttr(kNameSpaceID_None, nsGkAtoms::sbattr,
-                          NS_LITERAL_STRING("scrollbar-down-top"), false);
-  mDownTopButton->SetAttr(kNameSpaceID_None, nsGkAtoms::type,
-                          NS_LITERAL_STRING("increment"), false);
-
-  if (!aElements.AppendElement(mDownTopButton)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+  {
+    AnonymousContentKey key;
+    mDownTopButton =
+        MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ false,
+                            /* aDown */ true, key);
+    aElements.AppendElement(ContentInfo(mDownTopButton, key));
   }
 
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mSlider),
-      nodeInfoManager->GetNodeInfo(nsGkAtoms::slider, nullptr, kNameSpaceID_XUL,
-                                   nsINode::ELEMENT_NODE));
-  mSlider->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, orient, false);
-  mSlider->SetAttr(kNameSpaceID_None, nsGkAtoms::flex, NS_LITERAL_STRING("1"),
-                   false);
+  {
+    AnonymousContentKey key = AnonymousContentKey::Type_Slider;
+    if (vertical) {
+      key |= AnonymousContentKey::Flag_Vertical;
+    }
 
-  if (!aElements.AppendElement(mSlider)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+    NS_TrustedNewXULElement(
+        getter_AddRefs(mSlider),
+        nodeInfoManager->GetNodeInfo(nsGkAtoms::slider, nullptr,
+                                     kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
+    mSlider->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, orient, false);
+    mSlider->SetAttr(kNameSpaceID_None, nsGkAtoms::flex, NS_LITERAL_STRING("1"),
+                     false);
+
+    aElements.AppendElement(ContentInfo(mSlider, key));
+
+    NS_TrustedNewXULElement(
+        getter_AddRefs(mThumb),
+        nodeInfoManager->GetNodeInfo(nsGkAtoms::thumb, nullptr,
+                                     kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
+    mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, orient, false);
+    mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::align,
+                    NS_LITERAL_STRING("center"), false);
+    mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::pack,
+                    NS_LITERAL_STRING("center"), false);
+    mSlider->AppendChildTo(mThumb, false);
   }
 
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mThumb),
-      nodeInfoManager->GetNodeInfo(nsGkAtoms::thumb, nullptr, kNameSpaceID_XUL,
-                                   nsINode::ELEMENT_NODE));
-  mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, orient, false);
-  mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::align,
-                  NS_LITERAL_STRING("center"), false);
-  mThumb->SetAttr(kNameSpaceID_None, nsGkAtoms::pack,
-                  NS_LITERAL_STRING("center"), false);
-  mSlider->AppendChildTo(mThumb, false);
-
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mUpBottomButton),
-      nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbarbutton, nullptr,
-                                   kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
-  mUpBottomButton->SetAttr(kNameSpaceID_None, nsGkAtoms::type,
-                           NS_LITERAL_STRING("decrement"), false);
-  mUpBottomButton->SetAttr(kNameSpaceID_None, nsGkAtoms::sbattr,
-                           NS_LITERAL_STRING("scrollbar-up-bottom"), false);
-
-  if (!aElements.AppendElement(mUpBottomButton)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+  {
+    AnonymousContentKey key;
+    mUpBottomButton =
+        MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ true,
+                            /* aDown */ false, key);
+    aElements.AppendElement(ContentInfo(mUpBottomButton, key));
   }
 
-  NS_TrustedNewXULElement(
-      getter_AddRefs(mDownBottomButton),
-      nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbarbutton, nullptr,
-                                   kNameSpaceID_XUL, nsINode::ELEMENT_NODE));
-  mDownBottomButton->SetAttr(kNameSpaceID_None, nsGkAtoms::sbattr,
-                             NS_LITERAL_STRING("scrollbar-down-bottom"), false);
-  mDownBottomButton->SetAttr(kNameSpaceID_None, nsGkAtoms::type,
-                             NS_LITERAL_STRING("increment"), false);
+  {
+    AnonymousContentKey key;
+    mDownBottomButton =
+        MakeScrollbarButton(sbbNodeInfo, vertical, /* aBottom */ true,
+                            /* aDown */ true, key);
+    aElements.AppendElement(ContentInfo(mDownBottomButton, key));
+  }
 
-  if (!aElements.AppendElement(mDownBottomButton)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+  // Don't cache styles if we are inside a <select> element, since we have
+  // some UA style sheet rules that depend on the <select>'s attributes.
+  if (GetContent()->GetParent() &&
+      GetContent()->GetParent()->IsHTMLElement(nsGkAtoms::select)) {
+    for (auto& info : aElements) {
+      info.mKey = AnonymousContentKey::None;
+    }
   }
 
   UpdateChildrenAttributeValue(nsGkAtoms::curpos, false);

modules/libpref/init/StaticPrefList.h

@@ -4707,6 +4707,14 @@ VARCACHE_PREF(
   RelaxedAtomicUint32, 4096
 )
 
+// Is the codepath for using cached scrollbar styles enabled?
+VARCACHE_PREF(
+  Live,
+  "layout.css.cached-scrollbar-styles.enabled",
+  layout_css_cached_scrollbar_styles_enabled,
+  bool, false
+)
+
 // Is path() supported in clip-path?
 VARCACHE_PREF(
   Live,

servo/components/style/properties/properties.mako.rs

@@ -2836,6 +2836,19 @@ impl ComputedValues {
     pub fn resolve_color(&self, color: computed::Color) -> RGBA {
         color.to_rgba(self.get_inherited_text().clone_color())
     }
+
+    /// Returns which longhand properties have different values in the two
+    /// ComputedValues.
+    #[cfg(feature = "gecko_debug")]
+    pub fn differing_properties(&self, other: &ComputedValues) -> LonghandIdSet {
+        let mut set = LonghandIdSet::new();
+        % for prop in data.longhands:
+        if self.clone_${prop.ident}() != other.clone_${prop.ident}() {
+            set.insert(LonghandId::${prop.camel_case});
+        }
+        % endfor
+        set
+    }
 }
 
 #[cfg(feature = "servo")]

servo/ports/geckolib/glue.rs

@@ -3746,6 +3746,69 @@ pub extern "C" fn Servo_ComputedValues_GetStyleRuleList(
     }))
 }
 
+/// println_stderr!() calls Gecko's printf_stderr(), which, unlike eprintln!(),
+/// will funnel output to Android logcat.
+#[cfg(feature = "gecko_debug")]
+macro_rules! println_stderr {
+    ($($e:expr),+) => {
+        {
+            let mut s = nsCString::new();
+            write!(s, $($e),+).unwrap();
+            s.write_char('\n').unwrap();
+            unsafe { bindings::Gecko_PrintfStderr(&s); }
+        }
+    }
+}
+
+#[cfg(feature = "gecko_debug")]
+fn dump_properties_and_rules(cv: &ComputedValues, properties: &LonghandIdSet) {
+    println_stderr!("  Properties:");
+    for p in properties.iter() {
+        let mut v = String::new();
+        cv.get_longhand_property_value(p, &mut CssWriter::new(&mut v)).unwrap();
+        println_stderr!("    {:?}: {}", p, v);
+    }
+    println_stderr!("  Rules:");
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let guard = global_style_data.shared_lock.read();
+    for rn in cv.rules().self_and_ancestors() {
+        if rn.importance().important() {
+            continue;
+        }
+        if let Some(d) = rn.style_source().and_then(|s| s.as_declarations()) {
+            println_stderr!("    [DeclarationBlock: {:?}]", d);
+        }
+        if let Some(r) = rn.style_source().and_then(|s| s.as_rule()) {
+            let mut s = nsString::new();
+            r.read_with(&guard).to_css(&guard, &mut s).unwrap();
+            println_stderr!("    {}", s);
+        }
+    }
+}
+
+#[cfg(feature = "gecko_debug")]
+#[no_mangle]
+pub extern "C" fn Servo_ComputedValues_EqualForCachedAnonymousContentStyle(
+    a: &ComputedValues,
+    b: &ComputedValues,
+) -> bool {
+    let mut differing_properties = a.differing_properties(b);
+
+    // Ignore any difference in -x-lang, which we can't override in the
+    // rules in minimal-xul.css, but which makes no difference for the
+    // anonymous content subtrees we cache style for.
+    differing_properties.remove(LonghandId::XLang);
+
+    if !differing_properties.is_empty() {
+        println_stderr!("Actual style:");
+        dump_properties_and_rules(a, &differing_properties);
+        println_stderr!("Expected style:");
+        dump_properties_and_rules(b, &differing_properties);
+    }
+
+    differing_properties.is_empty()
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_Init(doc: &structs::Document) -> *mut RawServoStyleSet {
     let data = Box::new(PerDocumentStyleData::new(doc));

toolkit/content/minimal-xul.css

@@ -38,6 +38,61 @@
   visibility: collapse;
 }
 
+/* Rules required for style caching of anonymous content scrollbar parts */
+
+scrollcorner, resizer, scrollbar, scrollbarbutton, slider {
+  /* All scrollbar parts must not inherit any properties from the scrollable
+   * element (except for visibility), for the anonymous content style caching
+   * system to work.
+   */
+  all: initial;
+  visibility: inherit;
+
+  /* These properties are not included in 'all'. */
+  -moz-list-reversed: initial;
+  -moz-font-smoothing-background-color: initial;
+
+  /* Using initial is not sufficient for direction, since its initial value can
+   * depend on the document's language.
+   *
+   * LTR is what we want for all scrollbar parts anyway, so that e.g. we don't
+   * reverse the rendering of a horizontal scrollbar.
+   */
+  direction: ltr;
+
+  /* Similarly for font properties, whose initial values depend on the
+   * document's language.  Scrollbar parts don't have any text or rely on
+   * font metrics.
+   */
+  font: 16px sans-serif;
+
+  /* Duplicate the rules from the '*' rule above, which were clobbered by the
+   * 'all: initial' declaration.
+   *
+   * The other zero specificity rules above are on :root, and scrollbar parts
+   * cannot match :root, so no need to duplicate them.
+   */
+  -moz-user-focus: ignore;
+  -moz-user-select: none;
+  display: -moz-box;
+  box-sizing: border-box;
+}
+
+thumb {
+  /* Prevent -moz-user-modify declaration from designmode.css having an
+   * effect. */
+  -moz-user-modify: initial;
+}
+
+/* There are other rules that set direction and cursor on scrollbar,
+ * expecting them to inherit into its children.  Explicitly inherit those,
+ * overriding the 'all: initial; direction: ltr;' declarations above.
+ */
+scrollbarbutton, slider, thumb {
+  direction: inherit;
+  cursor: inherit;
+}
+
 /********** resizer **********/
 
 resizer {
@@ -97,11 +152,6 @@ resizer[dir="topright"] {
 
 /********** scrollbar **********/
 
-/* Scrollbars are never flipped even if BiDI kicks in. */
-scrollbar[orient="horizontal"] {
-  direction: ltr;
-}
-
 thumb {
   display: -moz-box !important;
 }