Prevent perpetual scroll.
Prevent perpetual scroll.

If we miss the keyup event while a smooth scroll is active (because the
focus changes), then we scroll forever.  This stops scrolling on blur.

Fixes #1788.

PrestanceDesign/vimium,Aleroniponi/vimium,mrmr1993/vimium,hhsue/vimium,Robinson10240/vimium,gdh1995/vimium,tuchangwei/vimium,csmalin/vimium,Lw-Cui/vimium,cyrixhero/vimium,smblott-github/vimium,poacher2k/vimium,willsALMANJ/vimium,csmalin/vimium,smblott-github/vimium,philc/vimium,willsALMANJ/vimium,borgified/vimium,Lw-Cui/vimium,zhangmin510/vimium,dimatter/vimium,elegantwww/vimium,decaffeinate-examples/vimium,poacher2k/vimium,borgified/vimium,mrmr1993/vimium,VPashkov/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,dimatter/vimium,csmalin/vimium,gdh1995/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,VPashkov/vimium,tuchangwei/vimium,PickRelated/vimium,Aleroniponi/vimium,hhsue/vimium,PrestanceDesign/vimium,cyrixhero/vimium,VPashkov/vimium,Robinson10240/vimium,philc/vimium,PickRelated/vimium,PrestanceDesign/vimium,philc/vimium,poacher2k/vimium,smblott-github/vimium,elegantwww/vimium,hhsue/vimium,elegantwww/vimium,borgified/vimium,Robinson10240/vimium,cyrixhero/vimium,zhangmin510/vimium,Lw-Cui/vimium,Aleroniponi/vimium,zhangmin510/vimium,PickRelated/vimium,gdh1995/vimium,dimatter/vimium,tuchangwei/vimium