Bug 1573014 - Early-exit ShrinkToDisplaySizeIfNeeded() in reader mode. r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D42967

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/test/gtest/mvm/TestMobileViewportManager.cpp

@@ -74,6 +74,7 @@ class MockMVMContext : public MVMContext {
     return Some(mDisplaySize);
   }
   bool AllowZoomingForDocument() const { return true; }
+  bool IsInReaderMode() const { return false; }
   bool IsDocumentLoading() const { return false; }
 
   void SetResolutionAndScaleTo(float aResolution,

layout/base/GeckoMVMContext.cpp

@@ -124,6 +124,16 @@ bool GeckoMVMContext::AllowZoomingForDocument() const {
   return nsLayoutUtils::AllowZoomingForDocument(mDocument);
 }
 
+bool GeckoMVMContext::IsInReaderMode() const {
+  MOZ_ASSERT(mDocument);
+  nsString uri;
+  if (NS_FAILED(mDocument->GetDocumentURI(uri))) {
+    return false;
+  }
+  static auto readerModeUriPrefix = NS_LITERAL_STRING("about:reader");
+  return StringBeginsWith(uri, readerModeUriPrefix);
+}
+
 bool GeckoMVMContext::IsDocumentLoading() const {
   MOZ_ASSERT(mDocument);
   return mDocument->GetReadyStateEnum() == Document::READYSTATE_LOADING;

layout/base/GeckoMVMContext.h

@@ -47,6 +47,7 @@ class GeckoMVMContext : public MVMContext {
       const override;
   Maybe<LayoutDeviceIntSize> GetContentViewerSize() const override;
   bool AllowZoomingForDocument() const override;
+  bool IsInReaderMode() const override;
   bool IsDocumentLoading() const override;
 
   void SetResolutionAndScaleTo(float aResolution,

layout/base/MVMContext.h

@@ -51,6 +51,7 @@ class MVMContext {
       const = 0;
   virtual Maybe<LayoutDeviceIntSize> GetContentViewerSize() const = 0;
   virtual bool AllowZoomingForDocument() const = 0;
+  virtual bool IsInReaderMode() const = 0;
   virtual bool IsDocumentLoading() const = 0;
 
   virtual void SetResolutionAndScaleTo(float aResolution,

layout/base/MobileViewportManager.cpp

@@ -614,10 +614,13 @@ void MobileViewportManager::ShrinkToDisplaySizeIfNeeded(
     return;
   }
 
-  if (!mContext->AllowZoomingForDocument()) {
+  if (!mContext->AllowZoomingForDocument() || mContext->IsInReaderMode()) {
     // If zoom is disabled, we don't scale down wider contents to fit them
     // into device screen because users won't be able to zoom out the tiny
     // contents.
+    // We special-case reader mode, because it doesn't allow zooming, but
+    // the restriction is often not yet in place at the time this logic
+    // runs.
     return;
   }