Improve scrolling while user has scrolled up
Improve scrolling while user has scrolled up

Fixes #953

akalipetis/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js