Code review comment for lp:~sil/gwibber/shorten-urls-async-521974

Insert a placeholder character when pasting a URL for shortening, and then asynchronously replace the placeholder character with the shortened URL.

Easier to test if your network connection is being slow, so go and download some films over bittorrent or something and then try pasting a long URL into the post entry and continuing to type while it's fetching the shortened version.