Bug 1611236 - Optimize adoptedStyleSheets append case r=emilio

- Generalize enumeration over unique sheets to take any array of sheets.
- Add check if the new adoptedStyleSheets only appends sheets.
- Handle append case for new adoptedStyleSheets.
- Add WPT test case for duplicate sheets in the ShadowRoot.
- Add WPT test cases for appending sheets.

Differential Revision: https://phabricator.services.mozilla.com/D64686

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

dom/base/DocumentOrShadowRoot.cpp

@@ -127,14 +127,50 @@ void DocumentOrShadowRoot::SetAdoptedStyleSheets(
   auto* shadow = ShadowRoot::FromNode(AsNode());
   MOZ_ASSERT((mKind == Kind::ShadowRoot) == !!shadow);
 
-  ClearAdoptedStyleSheets();
+  StyleSheetSet set(aAdoptedStyleSheets.Length());
+  size_t commonPrefix = 0;
+
+  // Find the index at which the new array differs from the old array.
+  // We don't want to do extra work for the sheets that both arrays have.
+  size_t min =
+      std::min(aAdoptedStyleSheets.Length(), mAdoptedStyleSheets.Length());
+  for (size_t i = 0; i < min; ++i) {
+    if (aAdoptedStyleSheets[i] != mAdoptedStyleSheets[i]) {
+      break;
+    }
+    ++commonPrefix;
+    set.PutEntry(mAdoptedStyleSheets[i]);
+  }
+
+  // Try to truncate the sheets to a common prefix.
+  // If the prefix contains duplicates of sheets that we are removing,
+  // we are just going to re-build everything from scratch.
+  if (commonPrefix != mAdoptedStyleSheets.Length()) {
+    StyleSheetSet removedSet(mAdoptedStyleSheets.Length() - commonPrefix);
+    for (size_t i = mAdoptedStyleSheets.Length(); i != commonPrefix; --i) {
+      RefPtr<StyleSheet> sheetToRemove = mAdoptedStyleSheets.PopLastElement();
+      if (MOZ_UNLIKELY(set.Contains(sheetToRemove))) {
+        // Fixing duplicate sheets would require insertions/removals from the
+        // style set. We may as well just rebuild the whole thing from scratch.
+        set.Clear();
+        // Note that setting this to zero means we'll continue the loop until
+        // all the sheets are cleared.
+        commonPrefix = 0;
+      }
+      if (MOZ_LIKELY(removedSet.EnsureInserted(sheetToRemove))) {
+        RemoveSheetFromStylesIfApplicable(*sheetToRemove);
+        sheetToRemove->RemoveAdopter(*this);
+      }
+    }
+    mAdoptedStyleSheets.TruncateLength(commonPrefix);
+  }
 
   // 3. Set the adopted style sheets to the new sheets
   mAdoptedStyleSheets.SetCapacity(aAdoptedStyleSheets.Length());
 
-  AdoptedStyleSheetSet set(aAdoptedStyleSheets.Length());
-  for (const OwningNonNull<StyleSheet>& sheet : aAdoptedStyleSheets) {
-    if (MOZ_UNLIKELY(!set.EnsureInserted(sheet.get()))) {
+  // Only add sheets that are not already in the common prefix.
+  for (const auto& sheet : MakeSpan(aAdoptedStyleSheets).From(commonPrefix)) {
+    if (MOZ_UNLIKELY(!set.EnsureInserted(sheet))) {
       // The idea is that this case is rare, so we pay the price of removing the
       // old sheet from the styles and append it later rather than the other way
       // around.

dom/base/DocumentOrShadowRoot.h

@@ -231,7 +231,7 @@ class DocumentOrShadowRoot {
   // with them.
   template <typename Callback>
   void EnumerateUniqueAdoptedStyleSheetsBackToFront(Callback aCallback) {
-    AdoptedStyleSheetSet set(mAdoptedStyleSheets.Length());
+    StyleSheetSet set(mAdoptedStyleSheets.Length());
     for (StyleSheet* sheet : Reversed(mAdoptedStyleSheets)) {
       if (MOZ_UNLIKELY(!set.EnsureInserted(sheet))) {
         continue;
@@ -241,7 +241,7 @@ class DocumentOrShadowRoot {
   }
 
  protected:
-  using AdoptedStyleSheetSet = nsTHashtable<nsPtrHashKey<const StyleSheet>>;
+  using StyleSheetSet = nsTHashtable<nsPtrHashKey<const StyleSheet>>;
   void RemoveSheetFromStylesIfApplicable(StyleSheet&);
   void ClearAdoptedStyleSheets();
 

testing/web-platform/tests/css/cssom/CSSStyleSheet-constructable-duplicate.html

@@ -4,17 +4,81 @@
 <link rel="help" href="https://wicg.github.io/construct-stylesheets/">
 <script src = '/resources/testharness.js'></script>
 <script src = '/resources/testharnessreport.js'></script>
-<div></div>
+<div id="target"></div>
 <script>
-test(function() {
-  let sheets = [];
 
-  for (let i = 0; i < 2; ++i) {
+function attachShadowDiv(host) {
+  const shadowRoot = host.attachShadow({mode: 'open'});
+  const shadowDiv = document.createElement("div");
+  shadowRoot.appendChild(shadowDiv);
+  return shadowDiv;
+}
+
+function blueSheetsWithIncreasingZIndex(n) {
+  let sheets = [];
+  for (let i = 0; i < n; ++i) {
     sheets.push(new CSSStyleSheet());
-    sheets[i].replaceSync("div { z-index: " + i + " }");
+    sheets[i].replaceSync("div { z-index: " + i + "; color: blue; }");
   }
+  return sheets;
+}
+
+let sheets = [];
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
 
   document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
   assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
-});
+
+  document.adoptedStyleSheets = [];
+}, "Duplicate stylesheets have the right cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take the right position in the cascade");
+  assert_equals(getComputedStyle(document.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+
+  document.adoptedStyleSheets = [];
+}, "Appending duplicate stylesheets yields the correct cascade position in the Document");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+}, "Duplicate stylesheets have the right cascade position in the ShadowRoot");
+
+test(function() {
+  sheets = blueSheetsWithIncreasingZIndex(2);
+
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync("div { color: red; }");
+
+  const span = document.createElement("span");
+  target.appendChild(span);
+  attachShadowDiv(span);
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0]];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "0", "backmost stylesheet should take precedence");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(0, 0, 255)", "backmost stylesheet should take precedence");
+
+  span.shadowRoot.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1], sheet];
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+  assert_equals(getComputedStyle(span.shadowRoot.querySelector("div")).color, "rgb(255, 0, 0)", "backmost stylesheet should take precedence");
+}, "Appending duplicate stylesheets yields the correct cascade position in the ShadowRoot");
+
 </script>