[show-hint addon] Fix race condition in scroll measurement

Issue #6656
Issue #6467

Author: marijnh

addon/hint/show-hint.js

@@ -322,6 +322,7 @@
 
     cm.on("scroll", this.onScroll = function() {
       var curScroll = cm.getScrollInfo(), editor = cm.getWrapperElement().getBoundingClientRect();
+      if (!startScroll) startScroll = cm.getScrollInfo();
       var newTop = top + startScroll.top - curScroll.top;
       var point = newTop - (parentWindow.pageYOffset || (ownerDocument.documentElement || ownerDocument.body).scrollTop);
       if (!below) point += hints.offsetHeight;