Use of many shadow trees with unique style triggers significant CPU u…

…sage and a rise in process size

https://bugs.webkit.org/show_bug.cgi?id=260925
rdar://114731415

Reviewed by Alan Baradlay.

Style resolver sharing cache in Style::Scope has unlimited size and the test case keeps adding more items
there. Low memory handler clears the cache causing memory use to eventually drop.

* Source/WebCore/dom/InlineStyleSheetOwner.cpp:
(WebCore::InlineStyleSheetOwner::createSheet):

Increase the size of the inline stylesheet sharing cache  (50 -> 256 items). Content that uses many unique
shadow tree style (like this test) could overrun this smallish cache easily rendering it ineffective.

Style resolver sharing in Style::Scope depends on inline stylesheet getting shared so this also made
that cache work poorly.

* Source/WebCore/style/StyleScope.cpp:
(WebCore::Style::Scope::createOrFindSharedShadowTreeResolver):

Add a limit to the resolver sharing cache size.

Canonical link: https://commits.webkit.org/269142@main

Source/WebCore/dom/InlineStyleSheetOwner.cpp

@@ -213,7 +213,7 @@ void InlineStyleSheetOwner::createSheet(Element& element, const String& text)
         inlineStyleSheetCache().add(*cacheKey, &m_sheet->contents());
 
         // Prevent pathological growth.
-        const size_t maximumInlineStyleSheetCacheSize = 50;
+        static constexpr auto maximumInlineStyleSheetCacheSize = 256;
         if (inlineStyleSheetCache().size() > maximumInlineStyleSheetCacheSize) {
             auto toRemove = inlineStyleSheetCache().random();
             toRemove->value->removedFromMemoryCache();

Source/WebCore/style/StyleScope.cpp

@@ -142,7 +142,12 @@ void Scope::createOrFindSharedShadowTreeResolver()
     if (!result.isNewEntry) {
         m_resolver = result.iterator->value.ptr();
         m_resolver->setSharedBetweenShadowTrees();
+        return;
     }
+
+    static constexpr auto maximimumSharedResolverCount = 256;
+    if (documentScope().m_sharedShadowTreeResolvers.size() > maximimumSharedResolverCount)
+        documentScope().m_sharedShadowTreeResolvers.remove(documentScope().m_sharedShadowTreeResolvers.random());
 }
 
 void Scope::unshareShadowTreeResolverBeforeMutation()