BL-599: Unresponsive script message while typing

Does the timeout happen on the same thread as keypresses? If not, we need to consider the possibility that by the time we get done updating the markup, the text has changed again. In such a case, this change would mean that the timeout that is active at the end of doMarkup would never be cleared. If the user types another keystroke before the timeout fires, both of them will happen. But I guess it must happen on the same thread, or our attempt to modify the text and reset the cursor would not work. In that case this line just prevents an unnecessary call to clearTimeout for a timeout that already happened, right?