Bug 1498812 - Part 1: Add helper method for directly retrieving the visual viewport's position. r=botond

Mainly required for testing.

Differential Revision: https://phabricator.services.mozilla.com/D15683

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

dom/base/nsDOMWindowUtils.cpp

@@ -1431,6 +1431,25 @@ nsDOMWindowUtils::GetVisualViewportOffsetRelativeToLayoutViewport(
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::GetVisualViewportOffset(int32_t* aOffsetX,
+                                          int32_t* aOffsetY) {
+  *aOffsetX = 0;
+  *aOffsetY = 0;
+
+  nsCOMPtr<Document> doc = GetDocument();
+  NS_ENSURE_STATE(doc);
+
+  nsIPresShell* presShell = doc->GetShell();
+  NS_ENSURE_TRUE(presShell, NS_ERROR_NOT_AVAILABLE);
+
+  nsPoint offset = presShell->GetVisualViewportOffset();
+  *aOffsetX = nsPresContext::AppUnitsToIntCSSPixels(offset.x);
+  *aOffsetY = nsPresContext::AppUnitsToIntCSSPixels(offset.y);
+
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDOMWindowUtils::GetScrollbarSize(bool aFlushLayout, int32_t* aWidth,
                                    int32_t* aHeight) {

dom/interfaces/base/nsIDOMWindowUtils.idl

@@ -848,6 +848,11 @@ interface nsIDOMWindowUtils : nsISupports {
   void getVisualViewportOffsetRelativeToLayoutViewport(out float aOffsetX,
                                                        out float aOffsetY);
 
+  /**
+   * Returns the scroll position of the window's visual viewport.
+   */
+  void getVisualViewportOffset(out long aOffsetX, out long aOffsetY);
+
   const long FLUSH_NONE = -1;
   const long FLUSH_STYLE = 0;
   const long FLUSH_LAYOUT = 1;