Bug 1423709 - Adapt initial zoom value to fit the wider contents to screen device width. r=botond

What we need to do here is to scale down the document as if the document was pinched in
to fit the contents to screen width when all the following conditions are met.

 - it's the first paint for the document
 - no initial-scale is specified in meta viewport tags
 - no mRestoreResolution hasn't set
 - the content width is wider than the initial viewport width

Depends on D10196

Differential Revision: https://phabricator.services.mozilla.com/D10197

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

layout/base/MobileViewportManager.cpp

@@ -233,13 +233,15 @@ MobileViewportManager::UpdateResolution(const nsViewportInfo& aViewportInfo,
   LayoutDeviceToLayerScale res(mPresShell->GetResolution());
 
   if (mIsFirstPaint) {
+    ScreenIntSize compositionSize = GetCompositionSize(aDisplaySize);
+
     CSSToScreenScale defaultZoom;
     if (mRestoreResolution) {
       LayoutDeviceToLayerScale restoreResolution(mRestoreResolution.value());
       if (mRestoreDisplaySize) {
         CSSSize prevViewport = mDocument->GetViewportInfo(mRestoreDisplaySize.value()).GetSize();
         float restoreDisplayWidthChangeRatio = (mRestoreDisplaySize.value().width > 0)
-          ? (float)aDisplaySize.width / (float)mRestoreDisplaySize.value().width : 1.0f;
+          ? (float)compositionSize.width / (float)mRestoreDisplaySize.value().width : 1.0f;
 
         restoreResolution =
           ScaleResolutionWithDisplayWidth(restoreResolution,
@@ -254,7 +256,9 @@ MobileViewportManager::UpdateResolution(const nsViewportInfo& aViewportInfo,
       defaultZoom = aViewportInfo.GetDefaultZoom();
       MVM_LOG("%p: default zoom from viewport is %f\n", this, defaultZoom.scale);
       if (!aViewportInfo.IsDefaultZoomValid()) {
-        defaultZoom = ComputeIntrinsicScale(aViewportInfo, aDisplaySize, aViewport);
+        defaultZoom = ComputeIntrinsicScale(aViewportInfo,
+                                            compositionSize,
+                                            aViewport);
       }
     }
     MOZ_ASSERT(aViewportInfo.GetMinZoom() <= defaultZoom &&
@@ -305,11 +309,11 @@ MobileViewportManager::UpdateResolution(const nsViewportInfo& aViewportInfo,
     PixelCastJustification::ScreenIsParentLayerForRoot);
 }
 
-void
-MobileViewportManager::UpdateVisualViewportSize(const ScreenIntSize& aDisplaySize,
-                                                const CSSToScreenScale& aZoom)
+
+ScreenIntSize
+MobileViewportManager::GetCompositionSize(const ScreenIntSize& aDisplaySize) const
 {
-  ScreenSize compositionSize(aDisplaySize);
+  ScreenIntSize compositionSize(aDisplaySize);
   ScreenMargin scrollbars =
     LayoutDeviceMargin::FromAppUnits(
       nsLayoutUtils::ScrollbarAreaToExcludeFromCompositionBoundsFor(
@@ -321,6 +325,16 @@ MobileViewportManager::UpdateVisualViewportSize(const ScreenIntSize& aDisplaySiz
 
   compositionSize.width -= scrollbars.LeftRight();
   compositionSize.height -= scrollbars.TopBottom();
+
+  return compositionSize;
+}
+
+void
+MobileViewportManager::UpdateVisualViewportSize(const ScreenIntSize& aDisplaySize,
+                                                const CSSToScreenScale& aZoom)
+{
+  ScreenSize compositionSize = ScreenSize(GetCompositionSize(aDisplaySize));
+
   CSSSize compSize = compositionSize / aZoom;
   MVM_LOG("%p: Setting VVPS %s\n", this, Stringify(compSize).c_str());
   nsLayoutUtils::SetVisualViewportSize(mPresShell, compSize);
@@ -444,7 +458,6 @@ MobileViewportManager::RefreshViewportSize(bool aForceAdjustResolution)
   CSSSize oldSize = mMobileViewportSize;
 
   // Update internal state.
-  mIsFirstPaint = false;
   mMobileViewportSize = viewport;
 
   // Kick off a reflow.
@@ -453,4 +466,44 @@ MobileViewportManager::RefreshViewportSize(bool aForceAdjustResolution)
     nsPresContext::CSSPixelsToAppUnits(viewport.height),
     nsPresContext::CSSPixelsToAppUnits(oldSize.width),
     nsPresContext::CSSPixelsToAppUnits(oldSize.height));
+
+  // We are going to fit the content to the display width if the initial-scale
+  // is not specied and if the content is still wider than the display width.
+  ShrinkToDisplaySizeIfNeeded(viewportInfo, displaySize);
+
+  mIsFirstPaint = false;
+}
+
+void
+MobileViewportManager::ShrinkToDisplaySizeIfNeeded(
+  nsViewportInfo& aViewportInfo,
+  const ScreenIntSize& aDisplaySize)
+{
+  if (!gfxPrefs::APZAllowZooming()) {
+    // If the APZ 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.
+    return;
+  }
+
+  // We try to scale down the contents only IF the document has no initial-scale
+  // AND IF it's the initial paint AND IF it's not restored documents.
+  if (aViewportInfo.IsDefaultZoomValid() ||
+      !mIsFirstPaint || mRestoreResolution) {
+    return;
+  }
+
+  nsIScrollableFrame* rootScrollableFrame =
+    mPresShell->GetRootScrollFrameAsScrollable();
+  if (rootScrollableFrame) {
+    nsRect scrollableRect =
+      nsLayoutUtils::CalculateScrollableRectForFrame(rootScrollableFrame,
+                                                     nullptr);
+    CSSSize contentSize = CSSSize::FromAppUnits(scrollableRect.Size());
+    CSSToScreenScale zoom =
+      UpdateResolution(aViewportInfo, aDisplaySize, contentSize, Nothing());
+
+    MVM_LOG("%p: Adapted zoom is %f\n", this, zoom.scale);
+    UpdateVisualViewportSize(aDisplaySize, zoom);
+  }
 }

layout/base/MobileViewportManager.h

@@ -110,6 +110,19 @@ public:
                                                   const mozilla::ScreenIntSize& aDisplaySize,
                                                   const mozilla::CSSSize& aViewportSize) const;
 
+  /*
+   * Returns the screen size subtracted the scrollbar sizes from |aDisplaySize|.
+   */
+  mozilla::ScreenIntSize
+  GetCompositionSize(const mozilla::ScreenIntSize& aDisplaySize) const;
+
+  /*
+   * Shrink the content to fit it to the display width if no initial-scale is
+   * specified and if the content is still wider than the display width.
+   */
+  void ShrinkToDisplaySizeIfNeeded(nsViewportInfo& aViewportInfo,
+                                   const mozilla::ScreenIntSize& aDisplaySize);
+
   nsCOMPtr<nsIDocument> mDocument;
   nsIPresShell* MOZ_NON_OWNING_REF mPresShell; // raw ref since the presShell owns this
   nsCOMPtr<mozilla::dom::EventTarget> mEventTarget;