I thought i'd already filed this (and it's quite similar to #6464 and #4708), but there's a really annoying failure mode where if you start typing a message and hit ctrl-A to select it in order to replace it with something else, you can end up with an uneditable character wedged at the beginning of the composer. If you keep ctrl-A-to-replacing then eventually you can get rid of it, but the race between moving the selection and typing the replacement is horrible and very similar to #4708.

Mainly filing the issue now to track whether switching to Slate fixes it (which superficially it does, so far, on the matthew/slate branch of react-sdk).