Bug 1000350. Don't overwrite a layout set display port base with an APZC-guessed one. r=kats

APZC set ones are only meant to fill in the gaps, use them only if we have nothing else.

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -244,7 +244,7 @@ APZCCallbackHelper::UpdateRootFrame(nsIDOMWindowUtils* aUtils,
                     baseCSS.y * nsPresContext::AppUnitsPerCSSPixel(),
                     baseCSS.width * nsPresContext::AppUnitsPerCSSPixel(),
                     baseCSS.height * nsPresContext::AppUnitsPerCSSPixel());
-        nsLayoutUtils::SetDisplayPortBase(content, base);
+        nsLayoutUtils::SetDisplayPortBaseIfNotSet(content, base);
     }
 }
 
@@ -299,7 +299,7 @@ APZCCallbackHelper::UpdateSubFrame(nsIContent* aContent,
                         baseCSS.y * nsPresContext::AppUnitsPerCSSPixel(),
                         baseCSS.width * nsPresContext::AppUnitsPerCSSPixel(),
                         baseCSS.height * nsPresContext::AppUnitsPerCSSPixel());
-            nsLayoutUtils::SetDisplayPortBase(aContent, base);
+            nsLayoutUtils::SetDisplayPortBaseIfNotSet(aContent, base);
         }
     }
 

layout/base/nsLayoutUtils.cpp

@@ -829,6 +829,14 @@ nsLayoutUtils::SetDisplayPortBase(nsIContent* aContent, const nsRect& aBase)
                         nsINode::DeleteProperty<nsRect>);
 }
 
+void
+nsLayoutUtils::SetDisplayPortBaseIfNotSet(nsIContent* aContent, const nsRect& aBase)
+{
+  if (!aContent->GetProperty(nsGkAtoms::DisplayPortBase)) {
+    SetDisplayPortBase(aContent, aBase);
+  }
+}
+
 bool
 nsLayoutUtils::GetCriticalDisplayPort(nsIContent* aContent, nsRect* aResult)
 {

layout/base/nsLayoutUtils.h

@@ -187,8 +187,11 @@ public:
   /**
    * Set the display port base rect for given element to be used with display
    * port margins.
+   * SetDisplayPortBaseIfNotSet is like SetDisplayPortBase except it only sets
+   * the display port base to aBase if no display port base is currently set.
    */
   static void SetDisplayPortBase(nsIContent* aContent, const nsRect& aBase);
+  static void SetDisplayPortBaseIfNotSet(nsIContent* aContent, const nsRect& aBase);
 
   /**
    * Get the critical display port for the given element.