Bug 773296 - Part 19: Compare style structs even for the same rule node when variables have changed. r=dbaron

This makes updates work correctly when variable values change.
Rather than handling nsStyleVariables with a DO_STRUCT_DIFFERENCE,
we explicitly compare the two nsStyleVariables objects in
nsStyleContext::CalcStyleDifference before looking at the other
style structs.  This is because we need to force those other
style structs to be compared if variable values are changing.
nsStyleVariables::CalcDifference still returns 0, since the change in
variable values themselves doesn't require any updates.

layout/style/CSSVariableValues.cpp

@@ -42,6 +42,28 @@ CSSVariableValues::operator=(const CSSVariableValues& aOther)
   return *this;
 }
 
+bool
+CSSVariableValues::operator==(const CSSVariableValues& aOther) const
+{
+  if (mVariables.Length() != aOther.mVariables.Length()) {
+    return false;
+  }
+
+  for (size_t thisIndex = 0; thisIndex < mVariables.Length(); ++thisIndex) {
+    size_t otherIndex;
+    if (!aOther.mVariableIDs.Get(mVariables[thisIndex].mVariableName,
+                                 &otherIndex)) {
+      return false;
+    }
+    const nsString& otherValue = aOther.mVariables[otherIndex].mValue;
+    if (!mVariables[thisIndex].mValue.Equals(otherValue)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 bool
 CSSVariableValues::Get(const nsAString& aName, nsString& aValue) const
 {

layout/style/CSSVariableValues.h

@@ -26,6 +26,10 @@ public:
 #endif
   CSSVariableValues& operator=(const CSSVariableValues& aOther);
 
+  bool operator==(const CSSVariableValues& aOther) const;
+  bool operator!=(const CSSVariableValues& aOther) const
+    { return !(*this == aOther); }
+
   /**
    * Gets the value of a variable in this set of computed variables.
    *

layout/style/nsStyleContext.cpp

@@ -440,7 +440,19 @@ nsStyleContext::CalcStyleDifference(nsStyleContext* aOther,
   // by font-size changing, so we don't need to worry about them like
   // we worry about 'inherit' values.)
   bool compare = mRuleNode != aOther->mRuleNode;
-  DebugOnly<int> styleStructCount = 0;
+
+  // If we had any change in variable values, then we'll need to examine
+  // all of the other style structs too, even if the new style context has
+  // the same rule node as the old one.
+  const nsStyleVariables* thisVariables = PeekStyleVariables();
+  if (thisVariables) {
+    const nsStyleVariables* otherVariables = aOther->StyleVariables();
+    if (thisVariables->mVariables != otherVariables->mVariables) {
+      compare = true;
+    }
+  }
+
+  DebugOnly<int> styleStructCount = 1;  // count Variables already
 
 #define DO_STRUCT_DIFFERENCE(struct_)                                         \
   PR_BEGIN_MACRO                                                              \
@@ -490,7 +502,6 @@ nsStyleContext::CalcStyleDifference(nsStyleContext* aOther,
   DO_STRUCT_DIFFERENCE(TextReset);
   DO_STRUCT_DIFFERENCE(Background);
   DO_STRUCT_DIFFERENCE(Color);
-  DO_STRUCT_DIFFERENCE(Variables);
 
 #undef DO_STRUCT_DIFFERENCE