Synched thumb nail page is not very much recognizable. So why don't you develop to show synched thumbnail page appear clearly
When user scroll up and down pages, thumb nail page in the left hand pane is not that much recognizable. I think system will be better showing thick solid line surrounded around the thumbnail page even user scroll the page without magnifying it. Hope this will make sense to you guys. It is simply dotted line surrounded, which is not recognizable.
Duckhwan Kim shared this idea