Easier to use shortcut for editing last sent message

[leoluk]

Most users are familiar with a simple Up shortcut for editing the last sent message since it's a common UI paradigm (Skype, Rocket Chat, readline-based CLI applications, ...).

I propose to support a similar shortcut in Zulip, since the Esc i Enter is error-prone and complicated. In our Zulip instance, users post messages containing a single "i" all the time when they get it wrong.

[timabbott]

Cool, thanks for the context! This is a duplicate of #1147 and is on our roadmap. See the discussion on that issue though about why Up might not be ideal -- feedback on the question of what hotkey to use is welcome!