Make mutation observer debounce at leading edge

src/annotator/plugin/cross-frame.coffee

@@ -68,7 +68,7 @@ module.exports = class CrossFrame extends Plugin
       _discoverOwnFrames()
 
       # Listen for DOM mutations, to know when frames are added / removed
-      observer = new MutationObserver(debounce(_discoverOwnFrames, 300))
+      observer = new MutationObserver(debounce(_discoverOwnFrames, 300, leading: true))
       observer.observe(elem, {childList: true, subtree: true});
 
     _discoverOwnFrames = ->