Bug 1500314 Part 1: Only apply mobile viewport stretching for documents with no meta viewport tag. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D23531

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

dom/base/Document.cpp

@@ -6627,7 +6627,11 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
     case Unknown: {
       nsAutoString viewport;
       GetHeaderData(nsGkAtoms::viewport, viewport);
-      if (viewport.IsEmpty()) {
+      // We might early exit if the viewport is empty. Even if we don't,
+      // at the end of this case we'll note that it was empty. Later, when
+      // we're using the cached values, this will trigger alternate code paths.
+      bool viewportIsEmpty = viewport.IsEmpty();
+      if (viewportIsEmpty) {
         // If the docType specifies that we are on a site optimized for mobile,
         // then we want to return specially crafted defaults for the viewport
         // info.
@@ -6721,11 +6725,12 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       mValidMaxScale =
           !maxScaleStr.IsEmpty() && NS_SUCCEEDED(scaleMaxErrorCode);
 
-      mViewportType = Specified;
+      mViewportType = viewportIsEmpty ? Empty : Specified;
       mViewportOverflowType = ViewportOverflowType::NoOverflow;
       MOZ_FALLTHROUGH;
     }
     case Specified:
+    case Empty:
     default:
       LayoutDeviceToScreenScale effectiveMinScale = mScaleMinFloat;
       LayoutDeviceToScreenScale effectiveMaxScale = mScaleMaxFloat;
@@ -6830,18 +6835,28 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       // https://drafts.csswg.org/css-device-adapt/#resolve-width
       if (width == nsViewportInfo::Auto) {
         if (height == nsViewportInfo::Auto || aDisplaySize.height == 0) {
-          // If we don't have any applicable viewport width constraints, this is
-          // most likely a desktop page written without mobile devices in mind.
-          // We use the desktop mode viewport for those pages by default,
-          // because a narrow viewport based on typical mobile device screen
-          // sizes (especially in portrait mode) will frequently break the
-          // layout of such pages. To keep text readable in that case, we rely
-          // on font inflation instead.
+          // What we do in this situation deviates somewhat from the spec. We
+          // want to consider the case where no viewport information has been
+          // provided, in which case we want to assume that the document is not
+          // optimized for aDisplaySize, and we should instead force a useful
+          // size.
+          if (mViewportType == Empty) {
+            // If we don't have any applicable viewport width constraints, this
+            // is most likely a desktop page written without mobile devices in
+            // mind. We use the desktop mode viewport for those pages by
+            // default, because a narrow viewport based on typical mobile device
+            // screen sizes (especially in portrait mode) will frequently break
+            // the layout of such pages. To keep text readable in that case, we
+            // rely on font inflation instead.
 
-          // Divide by fullZoom to stretch CSS pixel size of viewport in order
-          // to keep device pixel size unchanged after full zoom applied.
-          // See bug 974242.
-          width = gfxPrefs::DesktopViewportWidth() / fullZoom;
+            // Divide by fullZoom to stretch CSS pixel size of viewport in order
+            // to keep device pixel size unchanged after full zoom applied.
+            // See bug 974242.
+            width = gfxPrefs::DesktopViewportWidth() / fullZoom;
+          } else {
+            // Some viewport information was provided; follow the spec.
+            width = displaySize.width;
+          }
         } else {
           width = height * aDisplaySize.width / aDisplaySize.height;
         }

dom/base/Document.h

@@ -4480,7 +4480,7 @@ class Document : public nsINode,
   // Our update nesting level
   uint32_t mUpdateNestLevel;
 
-  enum ViewportType : uint8_t { DisplayWidthHeight, Specified, Unknown };
+  enum ViewportType : uint8_t { DisplayWidthHeight, Specified, Unknown, Empty };
 
   ViewportType mViewportType;