Bug 1194851. Introduce GetDisplayPortRelativeToScrollFrame. r=botond

layout/base/nsLayoutUtils.cpp

@@ -851,6 +851,21 @@ GetDisplayPortFromRectData(nsIContent* aContent,
   return ApplyRectMultiplier(aRectData->mRect, aMultiplier);
 }
 
+nsIFrame*
+GetScrollFrameFromContent(nsIContent* aContent)
+{
+  nsIFrame* frame = aContent->GetPrimaryFrame();
+  if (frame && aContent->OwnerDoc()->GetRootElement() == aContent) {
+    // We want the scroll frame, the root scroll frame differs from all
+    // others in that the primary frame is not the scroll frame.
+    if (nsIFrame* rootScrollFrame =
+          frame->PresContext()->PresShell()->GetRootScrollFrame()) {
+      frame = rootScrollFrame;
+    }
+  }
+  return frame;
+}
+
 static nsRect
 GetDisplayPortFromMarginsData(nsIContent* aContent,
                               DisplayPortMarginsPropertyData* aMarginsData,
@@ -869,7 +884,7 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
     // Fall through for graceful handling.
   }
 
-  nsIFrame* frame = aContent->GetPrimaryFrame();
+  nsIFrame* frame = GetScrollFrameFromContent(aContent);
   if (!frame) {
     // Turns out we can't really compute it. Oops. We still should return
     // something sane. Note that since we can't clamp the rect without a
@@ -881,13 +896,6 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
 
   bool isRoot = false;
   if (aContent->OwnerDoc()->GetRootElement() == aContent) {
-    // We want the scroll frame, the root scroll frame differs from all
-    // others in that the primary frame is not the scroll frame.
-    if (nsIFrame* rootScrollFrame =
-          frame->PresContext()->PresShell()->GetRootScrollFrame()) {
-      frame = rootScrollFrame;
-    }
-
     isRoot = true;
   }
 
@@ -1078,6 +1086,19 @@ nsLayoutUtils::GetDisplayPort(nsIContent* aContent, nsRect *aResult)
   return GetDisplayPortImpl(aContent, aResult, 1.0f);
 }
 
+bool
+nsLayoutUtils::GetDisplayPortRelativeToScrollFrame(nsIContent* aContent, nsRect *aResult)
+{
+  MOZ_ASSERT(aResult);
+  bool usingDisplayPort = GetDisplayPort(aContent, aResult);
+  nsIFrame* frame = GetScrollFrameFromContent(aContent);
+  nsIScrollableFrame* scrollableFrame = frame ? frame->GetScrollTargetFrame() : nullptr;
+  if (usingDisplayPort && scrollableFrame) {
+    *aResult += scrollableFrame->GetScrollPortRect().TopLeft();
+  }
+  return usingDisplayPort;
+}
+
 bool
 nsLayoutUtils::HasDisplayPort(nsIContent* aContent) {
   return GetDisplayPort(aContent, nullptr);

layout/base/nsLayoutUtils.h

@@ -166,10 +166,16 @@ public:
   static nsIScrollableFrame* FindScrollableFrameFor(ViewID aId);
 
   /**
-   * Get display port for the given element.
+   * Get display port for the given element. The displayport is relative to the
+   * scrollport.
    */
   static bool GetDisplayPort(nsIContent* aContent, nsRect *aResult);
 
+  /**
+   * Get display port for the given element relative to the scroll frame.
+   */
+  static bool GetDisplayPortRelativeToScrollFrame(nsIContent* aContent, nsRect *aResult);
+
   /**
    * Check whether the given element has a displayport.
    */