Restore prompt correctly when reconnecting in ERC

* lisp/erc/erc.el (erc--initialize-markers): Commit 0d6c8d41ab7 "Use
overlay instead of text prop to hide ERC's prompt" introduced a bug
that caused the prompt to remain hidden upon reconnecting because the
stashed overlay would get clobbered by ERC's major-mode setup.
Binding its old value while unhiding fixes the issue.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Don't permanently set
the default value of `erc-hide-prompt'.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--join-network-id): Add assertions for prompt
hiding and unhiding on reconnect.  (Bug#51082)

1 files changed, 3 insertions, 1 deletions

diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index a2f4562d333..616129bf780 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2394,7 +2394,9 @@ nil."
         (set-marker-insertion-type erc-insert-marker t)
         (cl-assert (= (field-end erc-insert-marker) erc-input-marker))
         (goto-char old-point)
-        (erc--unhide-prompt))
+        (let ((erc--hidden-prompt-overlay
+               (alist-get 'erc--hidden-prompt-overlay continued-session)))
+          (erc--unhide-prompt)))
     (cl-assert (not (get-text-property (point) 'erc-prompt)))
     ;; In the original version from `erc-open', the snippet that
     ;; handled these newline insertions appeared twice close in