The existing way to extend selection (discontinue selection) is move by ctrl+arrow and ctrl+space on each item. Ctrl+shift+arrow should be neat way to do this.
Created attachment 547564 [details] [diff] [review] fix + tests
BTW, there are test builds if you want to try it out: http://firstname.lastname@example.org/
Comment on attachment 547564 [details] [diff] [review] fix + tests r=me