Using the kakoune editor to compose a message, after a number of characters are inserted, the editor redraws itself over the entire aerc ui, escaping the embedded terminal's frame. After that happens, some features of the terminal stop working.
Tested in alacritty with TERM=xterm-256color
This also occurs on macOS, and definitely seems to be a bug with the embedded terminal emulator, since I don't see anything like this behavior in tmux or screen. In terms of severity, the rendering glitch is sufficiently total that aerc isn't actually usable with kakoune.