otherwise even using backspace in an annotated area will cause super slow
behaviour as each keystroke causes a full page render

This became a problem after

commit 0761f816
Date:   Thu Dec 19 18:50:58 2013 +0000
    123792: complete annotations on text ranges feature

but underlying problem was always there ready to trigger.

For this case only render the full page if the state
of comments scrollbars *toggles*, i.e. if there wasn't
scrollbars and there ends up still with no scrollbars
avoid the (bad) hack of invalidating the page

Change-Id: Ic7fd432d2317b0f5ad8d8773636f99fe75d660d6