Bug 1175940 - UIResolutionChanged should not trigger sync messages from content to chrome. r=mconley

dom/ipc/PBrowser.ipdl

@@ -723,8 +723,13 @@ child:
     /**
      * Tell the child that the UI resolution changed for the containing
      * window.
+     * To avoid some sync messages from child to parent, we also send the dpi
+     * and default scale with the notification.
+     * If we don't know the dpi and default scale, we just pass in a negative
+     * value (-1) but in the majority of the cases this saves us from two
+     * sync requests from the child to the parent.
      */
-    UIResolutionChanged();
+    UIResolutionChanged(float dpi, double scale);
 
     /**
      * Tell the child that the system theme has changed, and that a repaint

dom/ipc/TabChild.cpp

@@ -2904,12 +2904,12 @@ TabChild::RecvRequestNotifyAfterRemotePaint()
 }
 
 bool
-TabChild::RecvUIResolutionChanged()
+TabChild::RecvUIResolutionChanged(const float& aDpi, const double& aScale)
 {
   ScreenIntSize oldScreenSize = GetInnerSize();
   mDPI = 0;
   mDefaultScale = 0;
-  static_cast<PuppetWidget*>(mPuppetWidget.get())->ClearBackingScaleCache();
+  static_cast<PuppetWidget*>(mPuppetWidget.get())->UpdateBackingScaleCache(aDpi, aScale);
   nsCOMPtr<nsIDocument> document(GetDocument());
   nsCOMPtr<nsIPresShell> presShell = document->GetShell();
   if (presShell) {

dom/ipc/TabChild.h

@@ -472,7 +472,7 @@ public:
       return GetFrom(docShell);
     }
 
-    virtual bool RecvUIResolutionChanged() override;
+    virtual bool RecvUIResolutionChanged(const float& aDpi, const double& aScale) override;
 
     virtual bool RecvThemeChanged(nsTArray<LookAndFeelInt>&& aLookAndFeelIntCache) override;
 

dom/ipc/TabParent.cpp

@@ -1059,7 +1059,11 @@ TabParent::UIResolutionChanged()
     // mDPI being greater than 0, so this invalidates it.
     mDPI = -1;
     TryCacheDPIAndScale();
-    unused << SendUIResolutionChanged();
+    // If mDPI was set to -1 to invalidate it and then TryCacheDPIAndScale
+    // fails to cache the values, then mDefaultScale.scale might be invalid.
+    // We don't want to send that value to content. Just send -1 for it too in
+    // that case.
+    unused << SendUIResolutionChanged(mDPI, mDPI < 0 ? -1.0 : mDefaultScale.scale);
   }
 }
 

widget/PuppetWidget.h

@@ -196,10 +196,11 @@ public:
   virtual bool NeedsPaint() override;
 
   virtual TabChild* GetOwningTabChild() override { return mTabChild; }
-  virtual void ClearBackingScaleCache()
+
+  void UpdateBackingScaleCache(float aDpi, double aScale)
   {
-    mDPI = -1;
-    mDefaultScale = -1;
+    mDPI = aDpi;
+    mDefaultScale = aScale;
   }
 
   nsIntSize GetScreenDimensions();