I've somehow enabled overtype mode in Slack and I can't get back to insert mode
My original solution (crossed out below) no longer works. Things that do work:

If you have a Mac keyboard, press the Help key to toggle between insert and replace mode
If you have a Windows keyboard, press the Insert (or Ins) key to toggle between insert and replace mode
Restart Slack (quit and relaunch); refreshing no longer works

I contacted Slack support and was told to use ⌘+R to do a quick refresh and reset the mode from overwrite to insert.
From Slack support team:
&gt; Thanks for writing in about this! We've seen this occur on Mac computers recently as well, and the good news is there is an easy way to switch! To reset the mode from overwrite to insert, this should be possible with a quick refresh using Cmd+R
Explanation: This is because slack is using electron, which is essentially a wrapper around a browser to create a desktop app. Other &quot;web&quot; commands work like ⌘+[ and ⌘+] to traverse your &quot;browser history&quot; backward and forward, respectively.