bug#73863: 30.0.91; Unexpected cursor movement with flymake-show-diagnostics-at-end-of-line

[Eli Zaretskii <eliz@gnu.org>]

> From: Romain Ouabdelkader <romain.ouabdelkader@gmail.com>
> Date: Sat, 19 Oct 2024 15:41:05 +0200
> Cc: 73863@debbugs.gnu.org
> 
>  So this problem only happens when the window is not wide enough, and
>  the diagnostic wraps to the next screen line?
> 
> Yes that's correct, this happens to me quite often when I have two windows open side by side.

This is actually what flymake.el requests the display engine to do: it
asks it to show the cursor on the first character of the diagnostic's
before-string:

   (put-text-property 0 1 'cursor t summary)

So when you use C-n, Emacs moves point to the end of the
before-string, but the 'cursor' property tells it to show the cursor
on the first character of the before-string, which is on a previous
screen line.  So the display engine obeys and shows the cursor there.

The following simple patch seems to give much better results.  João,
is the code in question used by any Flymake feature other than this
new flymake-show-diagnostics-at-end-of-line option?  If so, we'll
probably need some condition to move the 'cursor' property to end of
string (assuming you don't object to this solution).

diff --git a/lisp/progmodes/flymake.el b/lisp/progmodes/flymake.el
index 9a6b62c..0ef1ed0 100644
--- a/lisp/progmodes/flymake.el
+++ b/lisp/progmodes/flymake.el
@@ -835,8 +835,12 @@ flymake--eol-overlay-summary
                                                (1- (length diags)))
                                        'face 'flymake-eol-information-face)))))
                    (t
-                    (mapconcat #'summarize diags " "))))))
-      (put-text-property 0 1 'cursor t summary)
+                    (mapconcat #'summarize diags " ")))))
+           (last-pos (1- (length summary))))
+      ;; Put the 'cursor' property on the last character of the summary,
+      ;; so that vertical-motion produces the expected results when the
+      ;; summary wraps to next screen lines.
+      (put-text-property last-pos (1+ last-pos) 'cursor t summary)
       summary)))
 
 (defun flymake--update-eol-overlays ()