Bug 1383367: Part 1 - Add JS helper to determine if a layout flush is required. r=mconley

This lets us decide whether to defer operations if they might trigger a
reflow.

MozReview-Commit-ID: 4M13HKAuZ7M

--HG--
extra : source : 6679237a46723432264361b5542454bb91d4831e
extra : intermediate-source : 46d1fa12a0829046f2bee4ffd10d7af38616bba9

dom/base/nsDOMWindowUtils.cpp

@@ -1825,6 +1825,39 @@ nsDOMWindowUtils::GetBoundsWithoutFlushing(nsIDOMElement *aElement,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::NeedsFlush(int32_t aFlushType, bool* aResult)
+{
+  MOZ_ASSERT(aResult);
+
+  nsCOMPtr<nsIDocument> doc = GetDocument();
+  NS_ENSURE_STATE(doc);
+
+  nsIPresShell* presShell = doc->GetShell();
+  NS_ENSURE_STATE(presShell);
+
+  FlushType flushType;
+  switch (aFlushType) {
+  case FLUSH_STYLE:
+    flushType = FlushType::Style;
+    break;
+
+  case FLUSH_LAYOUT:
+    flushType = FlushType::Layout;
+    break;
+
+  case FLUSH_DISPLAY:
+    flushType = FlushType::Display;
+    break;
+
+  default:
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = presShell->NeedFlush(flushType);
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDOMWindowUtils::GetRootBounds(nsIDOMClientRect** aResult)
 {

dom/interfaces/base/nsIDOMWindowUtils.idl

@@ -995,6 +995,15 @@ interface nsIDOMWindowUtils : nsISupports {
    */
   nsIDOMClientRect getBoundsWithoutFlushing(in nsIDOMElement aElement);
 
+  const long FLUSH_STYLE = 0;
+  const long FLUSH_LAYOUT = 1;
+  const long FLUSH_DISPLAY = 2;
+
+  /**
+   * Returns true if a flush of the given type is needed.
+   */
+  bool needsFlush(in long aFlushtype);
+
   /**
    * Returns the bounds of the window's currently loaded document. This will
    * generally be (0, 0, pageWidth, pageHeight) but in some cases (e.g. RTL