Bug 978913 - Dragging e10s window between HiDPI and non-HiDPI displays causes text to be scaled incorrectly. r=smaug.

dom/ipc/PBrowser.ipdl

@@ -503,6 +503,12 @@ child:
      */
     SetIsDocShellActive(bool aIsActive);
 
+    /**
+     * Tell the child that the UI resolution changed for the containing
+     * window.
+     */
+    UIResolutionChanged();
+
 /*
  * FIXME: write protocol!
 

dom/ipc/TabChild.cpp

@@ -2733,6 +2733,17 @@ TabChild::OnHideTooltip()
     return NS_OK;
 }
 
+bool
+TabChild::RecvUIResolutionChanged()
+{
+  static_cast<PuppetWidget*>(mWidget.get())->ClearBackingScaleCache();
+  nsCOMPtr<nsIDocument> document(GetDocument());
+  nsCOMPtr<nsIPresShell> presShell = document->GetShell();
+  nsRefPtr<nsPresContext> presContext = presShell->GetPresContext();
+  presContext->UIResolutionChanged();
+  return true;
+}
+
 TabChildGlobal::TabChildGlobal(TabChildBase* aTabChild)
 : mTabChild(aTabChild)
 {

dom/ipc/TabChild.h

@@ -456,6 +456,8 @@ public:
       return GetFrom(docShell);
     }
 
+    virtual bool RecvUIResolutionChanged() MOZ_OVERRIDE;
+
 protected:
     virtual PRenderFrameChild* AllocPRenderFrameChild(ScrollingBehavior* aScrolling,
                                                       TextureFactoryIdentifier* aTextureFactoryIdentifier,

dom/ipc/TabParent.cpp

@@ -509,6 +509,17 @@ TabParent::UpdateFrame(const FrameMetrics& aFrameMetrics)
   }
 }
 
+void
+TabParent::UIResolutionChanged()
+{
+  if (!mIsDestroyed) {
+    // TryCacheDPIAndScale()'s cache is keyed off of
+    // mDPI being greater than 0, so this invalidates it.
+    mDPI = -1;
+    unused << SendUIResolutionChanged();
+  }
+}
+
 void
 TabParent::AcknowledgeScrollUpdate(const ViewID& aScrollId, const uint32_t& aScrollGeneration)
 {

dom/ipc/TabParent.h

@@ -198,6 +198,7 @@ public:
     void Show(const nsIntSize& size);
     void UpdateDimensions(const nsRect& rect, const nsIntSize& size);
     void UpdateFrame(const layers::FrameMetrics& aFrameMetrics);
+    void UIResolutionChanged();
     void AcknowledgeScrollUpdate(const ViewID& aScrollId, const uint32_t& aScrollGeneration);
     void HandleDoubleTap(const CSSPoint& aPoint,
                          int32_t aModifiers,

layout/base/nsPresContext.cpp

@@ -49,11 +49,14 @@
 #include "mozilla/dom/MediaQueryList.h"
 #include "nsSMILAnimationController.h"
 #include "mozilla/css/ImageLoader.h"
+#include "mozilla/dom/PBrowserParent.h"
 #include "mozilla/dom/TabChild.h"
+#include "mozilla/dom/TabParent.h"
 #include "nsRefreshDriver.h"
 #include "Layers.h"
 #include "nsIDOMEvent.h"
 #include "gfxPrefs.h"
+#include "nsIDOMChromeWindow.h"
 
 #include "nsContentUtils.h"
 #include "nsCxPusher.h"
@@ -1768,6 +1771,35 @@ nsPresContext::UIResolutionChangedInternal()
     AppUnitsPerDevPixelChanged();
   }
 
+  nsCOMPtr<nsIDOMChromeWindow> chromeWindow(do_QueryInterface(mDocument->GetWindow()));
+  nsCOMPtr<nsIMessageBroadcaster> windowMM;
+  if (chromeWindow) {
+    chromeWindow->GetMessageManager(getter_AddRefs(windowMM));
+  }
+  if (windowMM) {
+    uint32_t tabChildCount = 0;
+    windowMM->GetChildCount(&tabChildCount);
+    for (uint32_t j = 0; j < tabChildCount; ++j) {
+      nsCOMPtr<nsIMessageListenerManager> childMM;
+      windowMM->GetChildAt(j, getter_AddRefs(childMM));
+      if (!childMM) {
+        continue;
+      }
+      nsCOMPtr<nsIMessageSender> tabMM = do_QueryInterface(childMM);
+
+      mozilla::dom::ipc::MessageManagerCallback* cb =
+       static_cast<nsFrameMessageManager*>(tabMM.get())->GetCallback();
+      if (cb) {
+        nsFrameLoader* fl = static_cast<nsFrameLoader*>(cb);
+        PBrowserParent* remoteBrowser = fl->GetRemoteBrowser();
+        TabParent* remote = static_cast<TabParent*>(remoteBrowser);
+        if (remote) {
+          remote->UIResolutionChanged();
+        }
+      }
+    }
+  }
+
   mDocument->EnumerateSubDocuments(UIResolutionChangedSubdocumentCallback,
                                    nullptr);
 }

widget/xpwidgets/PuppetWidget.h

@@ -183,6 +183,11 @@ public:
   virtual bool NeedsPaint() MOZ_OVERRIDE;
 
   virtual TabChild* GetOwningTabChild() MOZ_OVERRIDE { return mTabChild; }
+  virtual void ClearBackingScaleCache()
+  {
+    mDPI = -1;
+    mDefaultScale = -1;
+  }
 
 private:
   nsresult Paint();