Auto merge of #15837 - upsuper:bug1344135, r=emilio

Return true in set_property only when declaration block is changed

This is [bug 1344135](https://bugzilla.mozilla.org/show_bug.cgi?id=1344135).

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components/script/dom/cssstyledeclaration.rs

@@ -266,11 +266,9 @@ impl CSSStyleDeclaration {
 
             // Step 8
             // Step 9
-            // We could try to be better I guess?
+            *changed = false;
-            *changed = !declarations.is_empty();
             for declaration in declarations {
-                // TODO(emilio): We could check it changed
+                *changed |= pdb.set_parsed_declaration(declaration.0, importance);
-                pdb.set_parsed_declaration(declaration.0, importance);
             }
 
             Ok(())

components/style/properties/declaration_block.rs

@@ -171,10 +171,11 @@ impl PropertyDeclarationBlock {
     }
 
     /// Adds or overrides the declaration for a given property in this block,
-    /// without taking into account any kind of priority.
+    /// without taking into account any kind of priority. Returns whether the
+    /// declaration block is actually changed.
     pub fn set_parsed_declaration(&mut self,
                                   declaration: PropertyDeclaration,
-                                  importance: Importance) {
+                                  importance: Importance) -> bool {
         for slot in &mut *self.declarations {
             if slot.0.id() == declaration.id() {
                 match (slot.1, importance) {
@@ -184,10 +185,12 @@ impl PropertyDeclarationBlock {
                     (Importance::Important, Importance::Normal) => {
                         self.important_count -= 1;
                     }
-                    _ => {}
+                    _ => if slot.0 == declaration {
+                        return false;
+                    }
                 }
                 *slot = (declaration, importance);
-                return
+                return true;
             }
         }
 
@@ -195,6 +198,7 @@ impl PropertyDeclarationBlock {
         if importance.important() {
             self.important_count += 1;
         }
+        true
     }
 
     /// Set the declaration importance for a given property, if found.

ports/geckolib/glue.rs

@@ -837,10 +837,11 @@ fn set_property(declarations: RawServoDeclarationBlockBorrowed, property_id: Pro
                                              Box::new(StdoutErrorReporter), extra_data) {
         let mut declarations = RwLock::<PropertyDeclarationBlock>::as_arc(&declarations).write();
         let importance = if is_important { Importance::Important } else { Importance::Normal };
+        let mut changed = false;
         for decl in decls.into_iter() {
-            declarations.set_parsed_declaration(decl.0, importance);
+            changed |= declarations.set_parsed_declaration(decl.0, importance);
         }
-        true
+        changed
     } else {
         false
     }