bug#63829: 29.0.90; project-find-file's future history breaks with common-parent-directory

[Spencer Baugh <sbaugh@janestreet.com>]

Dmitry Gutov <dmitry@gutov.dev> writes:
> Try the patch at the end, please. It seems to fix the scenario you
> presented. Does it help with the feature you mention below, too?

Yes, your patch works great and solves the cases I care about!

On top of your patch, I can implement the feature I mentioned with the
patch at the end.  This causes the following nice behavior:

1. Open ~/src/emacs/emacs-29/lisp/progmodes/project.el
2. C-x p p ~/src/emacs/trunk
3. f
4. M-n and the minibuffer contains "lisp/progmodes/project.el"
5. RET and we have now easily switched to the same file in another
   project

Does the implementation seem OK?

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index ac7be8dcbb2..b1e01df5314 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1008,7 +1008,12 @@ project-find-file
          (dirs (list root)))
     (project-find-file-in
      (or (thing-at-point 'filename)
-         buffer-file-name)
+         (and buffer-file-name
+              (if-let (buffer-proj (and project-current-directory-override
+                                       (project-current nil default-directory)))
+                  (let ((buffer-root (project-root buffer-proj)))
+                    (file-name-concat root (file-relative-name buffer-file-name buffer-root)))
+                buffer-file-name)))
      dirs pr include-all)))
 
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