Bug 1507279 - Expose the visual scrolling mechanism to internal JS users. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D16142

dom/base/nsDOMWindowUtils.cpp

@@ -1396,6 +1396,23 @@ nsDOMWindowUtils::GetScrollXYFloat(bool aFlushLayout, float* aScrollX,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::ScrollToVisual(float aOffsetX, float aOffsetY) {
+  nsCOMPtr<Document> doc = GetDocument();
+  NS_ENSURE_STATE(doc);
+
+  nsPresContext* presContext = doc->GetPresContext();
+  NS_ENSURE_TRUE(presContext, NS_ERROR_NOT_AVAILABLE);
+
+  // This should only be called on the root content document.
+  NS_ENSURE_TRUE(presContext->IsRootContentDocument(), NS_ERROR_INVALID_ARG);
+
+  presContext->PresShell()->SetPendingVisualViewportOffset(
+      Some(CSSPoint::ToAppUnits(CSSPoint(aOffsetX, aOffsetY))));
+
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDOMWindowUtils::GetVisualViewportOffsetRelativeToLayoutViewport(
     float* aOffsetX, float* aOffsetY) {

dom/interfaces/base/nsIDOMWindowUtils.idl

@@ -828,6 +828,19 @@ interface nsIDOMWindowUtils : nsISupports {
    */
   DOMRect getBoundsWithoutFlushing(in Element aElement);
 
+  /**
+   * Scroll the visual viewport to the given coordinates, relative to the
+   * document origin.
+   * Only applicable to the window associated with the root content document.
+   * Note: this does not take effect right away. Rather, the visual scroll
+   *       request is sent to APZ with the next transaction, and will be
+   *       reflected in the main thread with the subsequent APZ repaint request.
+   * Please see the caveats mentioned at nsIPresShell::
+   * SetPendingVisualViewportOffset(), and request APZ review if adding a new
+   * call to this.
+   */
+  void scrollToVisual(in float aOffsetX, in float aOffsetY);
+ 
   /**
    * Returns the offset of the window's visual viewport relative to the
    * layout viewport.