Bug 1396323 - Don't initialize HTMLInputElement editor for eVoidEvent. r=ehsan

Keyboard APZ dispatches an eVoidEvent to gather all event targets that a key event
would normally go to. This can sometimes trigger an HTMLInputElement to initialize its
editor, which can cause unnecessary DOM modifications.

MozReview-Commit-ID: 6EEttouVB81

--HG--
extra : rebase_source : e7ff9c50a13f43f66976e3f0294e3d1696b87169

dom/html/HTMLInputElement.cpp

@@ -3572,13 +3572,17 @@ HTMLInputElement::NeedToInitializeEditorForEvent(
   // are lazily initialized.  We don't need to initialize the control for
   // certain types of events, because we know that those events are safe to be
   // handled without the editor being initialized.  These events include:
-  // mousein/move/out, overflow/underflow, and DOM mutation events.
+  // mousein/move/out, overflow/underflow, DOM mutation, and void events. Void
+  // events are dispatched frequently by async keyboard scrolling to focused
+  // elements, so it's important to handle them to prevent excessive DOM
+  // mutations.
   if (!IsSingleLineTextControl(false) ||
       aVisitor.mEvent->mClass == eMutationEventClass) {
     return false;
   }
 
   switch (aVisitor.mEvent->mMessage) {
+  case eVoidEvent:
   case eMouseMove:
   case eMouseEnterIntoWidget:
   case eMouseExitFromWidget: