The following commit introduced a regression:

  commit 05f6fdb9e7893329baff675bd31fb36ad64c756d
  Author: F. Jason Park <jp@neverwas.me>

  Preserve ERC prompt and its bounding markers

  1 file changed, 27 insertions(+), 22 deletions(-)
  lisp/erc/erc.el | 49 +++++++++++++++++++++++++++----------------------


To reproduce from emacs -Q:

  1. Eval:

     (require 'erc)
     (setq erc-prompt (lambda () (format-time-string "%T>"))
           erc-autojoin-channels-alist '((ErgoTestnet "#test")))
     (erc-tls :server "testnet.ergo.chat")

  2. In #test, note the timestamp in the prompt
  3. Say "something" RET
  4. Notice that the prompt doesn't change, whereas in ERC 5.5 and
     earlier, it would change on every outgoing message

The attached patch fixes the regression and changes the behavior to
redraw the prompt on every incoming message as well, but only when
`erc-prompt' is a function. Doing this should bring us one step closer
to being able to look at

  bug#51082 erc-prompt: support substitution patterns "%target" and "%network"     

However, we'd still be missing user-mode tracking, which seems fairly
trivial to add.