Bug 1556556 - Add ViewportUtils::IsZoomedContentRoot(). r=kats,mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D68727

layout/base/ViewportUtils.cpp

@@ -3,12 +3,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/PresShell.h"
+#include "mozilla/ViewportFrame.h"
 #include "mozilla/ViewportUtils.h"
 #include "mozilla/layers/APZCCallbackHelper.h"
 #include "mozilla/layers/InputAPZContext.h"
 #include "mozilla/layers/ScrollableLayerGuid.h"
 #include "nsIContent.h"
+#include "nsIFrame.h"
+#include "nsIScrollableFrame.h"
 #include "nsLayoutUtils.h"
+#include "nsQueryFrame.h"
+#include "nsStyleStruct.h"
 
 namespace mozilla {
 
@@ -117,4 +122,25 @@ template LayoutDeviceToLayoutDeviceMatrix4x4
     ViewportUtils::GetVisualToLayoutTransform<LayoutDevicePixel>(
         ScrollableLayerGuid::ViewID);
 
+const nsIFrame* ViewportUtils::IsZoomedContentRoot(const nsIFrame* aFrame) {
+  if (!aFrame) {
+    return nullptr;
+  }
+  if (aFrame->Type() == LayoutFrameType::Canvas) {
+    nsIScrollableFrame* sf = do_QueryFrame(aFrame->GetParent());
+    if (sf && sf->IsRootScrollFrameOfDocument() &&
+        aFrame->PresContext()->IsRootContentDocumentCrossProcess()) {
+      return aFrame->GetParent();
+    }
+  } else if (aFrame->StyleDisplay()->mPosition ==
+             StylePositionProperty::Fixed) {
+    if (ViewportFrame* viewportFrame = do_QueryFrame(aFrame->GetParent())) {
+      if (viewportFrame->PresContext()->IsRootContentDocumentCrossProcess()) {
+        return viewportFrame->PresShell()->GetRootScrollFrame();
+      }
+    }
+  }
+  return nullptr;
+}
+
 }  // namespace mozilla

layout/base/ViewportUtils.h

@@ -50,6 +50,17 @@ class ViewportUtils {
   static nsPoint VisualToLayout(const nsPoint& aPt, PresShell* aContext);
   static nsRect VisualToLayout(const nsRect& aRect, PresShell* aContext);
   static nsPoint LayoutToVisual(const nsPoint& aPt, PresShell* aContext);
+
+  /**
+   * Returns non-null if |aFrame| is inside the async zoom container but its
+   * parent frame is not, thereby making |aFrame| a root of a subtree of
+   * frames representing content that is zoomed in. The value returned in such
+   * cases is the root scroll frame inside the async zoom container.
+
+   * Callers use this to identify points during frame tree traversal where the
+   * visual-to-layout transform needs to be applied.
+   */
+  static const nsIFrame* IsZoomedContentRoot(const nsIFrame* aFrame);
 };
 
 // Forward declare explicit instantiations of GetVisualToLayoutTransform() for