Bug 1620778 - Fix interaction of up/down keys with autocomplete and <input type=number>. r=masayuki,smaug

Differential Revision: https://phabricator.services.mozilla.com/D66011

dom/html/TextControlState.cpp

@@ -950,6 +950,15 @@ TextInputListener::HandleEvent(Event* aEvent) {
     return NS_ERROR_UNEXPECTED;
   }
 
+  {
+    auto* input = HTMLInputElement::FromNode(mTxtCtrlElement);
+    if (input && input->StepsInputValue(*widgetKeyEvent)) {
+      // As an special case, don't handle key events that would step the value
+      // of our <input type=number>.
+      return NS_OK;
+    }
+  }
+
   KeyEventHandler* keyHandlers = ShortcutKeys::GetHandlers(
       mTxtCtrlElement->IsTextArea() ? HandlerType::eTextArea
                                     : HandlerType::eInput);