Bug 942749 - [keyboard] Fix forms.js contenteditable scroll position. r=fabrice

dom/inputmethod/forms.js

@@ -415,7 +415,7 @@ let FormAssistant = {
           this.scrollIntoViewTimeout = content.setTimeout(function () {
             this.scrollIntoViewTimeout = null;
             if (this.focusedElement && !FormVisibility.isVisible(this.focusedElement)) {
-              this.focusedElement.scrollIntoView(false);
+              scrollSelectionOrElementIntoView(this.focusedElement);
             }
           }.bind(this), RESIZE_SCROLL_DELAY);
         }
@@ -1017,6 +1017,23 @@ function setSelectionRange(element, start, end) {
   }
 }
 
+/**
+ * Scroll the given element into view.
+ *
+ * Calls scrollSelectionIntoView for contentEditable elements.
+ */
+function scrollSelectionOrElementIntoView(element) {
+  let editor = getPlaintextEditor(element);
+  if (editor) {
+    editor.selectionController.scrollSelectionIntoView(
+      Ci.nsISelectionController.SELECTION_NORMAL,
+      Ci.nsISelectionController.SELECTION_FOCUS_REGION,
+      Ci.nsISelectionController.SCROLL_SYNCHRONOUS);
+  } else {
+      element.scrollIntoView(false);
+  }
+}
+
 // Get nsIPlaintextEditor object from an input field
 function getPlaintextEditor(element) {
   let editor = null;