Thumbnail view improvement

thumbnail view needs some improvements:

1- speed is very slow specially if you quickly scroll to the bottom.
2- when you quickly scroll down to a position, it’s no need to render all previous pages. now it does so. but simply render the visible ones.
3- when scroll in thumbnails to some position (for example end of document) and close the thumbnail view and continue to read and research, if you refer to thumbnails it is still at the previous position (end of document in our example) not the current position. while the normal behavior is that whenever we open the thumbnail view, we can see the current page highlighted.

these are very important cases for make the thumbnail navigation useful.