Bug 1280013 - Incorporate the APZ callback transforms when using the root composition bounds to clip the displayport base. r=botond,tnikkel

MozReview-Commit-ID: 1HOGQnCjIMt

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -400,8 +400,8 @@ APZCCallbackHelper::ApplyCallbackTransform(const CSSPoint& aInput,
     // that's not on the Root Document (RD). That is, on platforms where
     // RCD == RD, it's 1, and on platforms where RCD != RD, it's the RCD
     // resolution. 'input' has this resolution applied, but the scroll
-    // deltas retrieved below do not, so we need to apply them to the
-    // deltas before adding the deltas to 'input'. (Technically, deltas
+    // delta retrieved below do not, so we need to apply them to the
+    // delta before adding the delta to 'input'. (Technically, deltas
     // from scroll frames outside the RCD would already have this
     // resolution applied, but we don't have such scroll frames in
     // practice.)
@@ -409,30 +409,10 @@ APZCCallbackHelper::ApplyCallbackTransform(const CSSPoint& aInput,
     if (nsIPresShell* shell = GetRootContentDocumentPresShellForContent(content)) {
       nonRootResolution = shell->GetCumulativeNonRootScaleResolution();
     }
-    // Now apply the callback-transform.
-    // XXX: Walk up the frame tree from the frame of this content element
-    // to the root of the frame tree, and apply any apzCallbackTransform
-    // found on the way. This is only approximately correct, as it does
-    // not take into account CSS transforms, nor differences in structure between
-    // the frame tree (which determines the transforms we're applying)
-    // and the layer tree (which determines the transforms we *want* to
-    // apply).
-    nsIFrame* frame = content->GetPrimaryFrame();
-    nsCOMPtr<nsIContent> lastContent;
-    while (frame) {
-        if (content && (content != lastContent)) {
-            void* property = content->GetProperty(nsGkAtoms::apzCallbackTransform);
-            if (property) {
-                CSSPoint delta = (*static_cast<CSSPoint*>(property));
-                delta = delta * nonRootResolution;
-                input += delta;
-            }
-        }
-        frame = frame->GetParent();
-        lastContent = content;
-        content = frame ? frame->GetContent() : nullptr;
-    }
-    return input;
+    // Now apply the callback-transform. This is only approximately correct,
+    // see the comment on GetCumulativeApzCallbackTransform for details.
+    CSSPoint transform = nsLayoutUtils::GetCumulativeApzCallbackTransform(content->GetPrimaryFrame());
+    return input + transform * nonRootResolution;
 }
 
 LayoutDeviceIntPoint

layout/base/nsLayoutUtils.cpp

@@ -9330,3 +9330,26 @@ nsLayoutUtils::IsTransformed(nsIFrame* aForFrame, nsIFrame* aTopFrame)
   return false;
 }
 
+/*static*/ CSSPoint
+nsLayoutUtils::GetCumulativeApzCallbackTransform(nsIFrame* aFrame)
+{
+  CSSPoint delta;
+  if (!aFrame) {
+    return delta;
+  }
+  nsIFrame* frame = aFrame;
+  nsCOMPtr<nsIContent> content = frame->GetContent();
+  nsCOMPtr<nsIContent> lastContent;
+  while (frame) {
+    if (content && (content != lastContent)) {
+      void* property = content->GetProperty(nsGkAtoms::apzCallbackTransform);
+      if (property) {
+        delta += *static_cast<CSSPoint*>(property);
+      }
+    }
+    frame = GetCrossDocParentFrame(frame);
+    lastContent = content;
+    content = frame ? frame->GetContent() : nullptr;
+  }
+  return delta;
+}

layout/base/nsLayoutUtils.h

@@ -2836,6 +2836,16 @@ public:
    */
   static bool IsTransformed(nsIFrame* aForFrame, nsIFrame* aTopFrame = nullptr);
 
+  /**
+   * Walk up from aFrame to the cross-doc root, accumulating all the APZ callback
+   * transforms on the content elements encountered along the way. Return the
+   * accumulated value.
+   * XXX: Note that this does not take into account CSS transforms, nor
+   * differences in structure between the frame tree and the layer tree (which
+   * is probably what we *want* to be computing).
+   */
+  static CSSPoint GetCumulativeApzCallbackTransform(nsIFrame* aFrame);
+
 private:
   static uint32_t sFontSizeInflationEmPerLine;
   static uint32_t sFontSizeInflationMinTwips;

layout/generic/nsGfxScrollFrame.cpp

@@ -3534,7 +3534,20 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
                 rootCompBounds = rootCompBounds.RemoveResolution(rootPresShell->GetResolution());
               }
 
+              // We want to convert the root composition bounds from the coordinate
+              // space of |rootFrame| to the coordinate space of |mOuter|. We do
+              // that with the TransformRect call below. However, since we care
+              // about the root composition bounds relative to what the user is
+              // actually seeing, we also need to incorporate the APZ callback
+              // transforms into this. Most of the time those transforms are
+              // negligible, but in some cases (e.g. when a zoom is applied on
+              // an overflow:hidden document) it is not (see bug 1280013).
+              // XXX: Eventually we may want to create a modified version of
+              // TransformRect that includes the APZ callback transforms
+              // directly.
               nsLayoutUtils::TransformRect(rootFrame, mOuter, rootCompBounds);
+              rootCompBounds += CSSPoint::ToAppUnits(
+                  nsLayoutUtils::GetCumulativeApzCallbackTransform(mOuter));
 
               displayportBase = displayportBase.Intersect(rootCompBounds);
             }