bug#45837: 28.0.50; incorrect cursor position in visual-line-mode when word-wrap-by-category is t

[Eli Zaretskii]

> From: Liu Hui <liuhui1610@gmail.com>
> Date: Thu, 14 Jan 2021 12:51:12 +0800
> Cc: 45837@debbugs.gnu.org
> 
> BTW, in the case above, if the line wraps after a non-whitespace
> character, C-k does not delete this character. How about the following
> change to kill-visual-line?

Yes, good catch.  However, this is not entirely right, as the code you
suggest should be _instead_ of skipping the whitespace that follows,
not _in_addition_ to it (imagine that the line is wrapped at a
non-whitespace character followed by whitespace).  And while looking
into adapting your patch to avoid removing more than the user
intended, I found more issues with the existing code.  So I'm
proposing the patch below instead, which should correctly handle the
following use cases:

 . visual line that ends in one or more whitespace characters, and the
   following visual line begins with one or more whitespace characters
 . visual line that ends with a non-whitespace character (under
   word-wrap-by-category) that is followed by one or more whitespace
   characters
 . line that is continued on the next visual line (visual-line-mode is
   off)
 . visual line that is wrapped in the middle of a display string or an
   overlay string with embedded whitespace characters
 . visual line that is wrapped in the middle of intangible text

(The patch also fixes some minor issues with the documentation of this
command.)

WDYT?

diff --git a/lisp/simple.el b/lisp/simple.el
index 54c35c04be..e7421c069f 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -5606,7 +5606,9 @@ zap-to-char
 ;; kill-line and its subroutines.
 
 (defcustom kill-whole-line nil
-  "If non-nil, `kill-line' with no arg at start of line kills the whole line."
+  "If non-nil, `kill-line' with no arg at start of line kills the whole line.
+This variable also affects `kill-visual-line' in the same way as
+it does `kill-line'."
   :type 'boolean
   :group 'killing)
 
@@ -7319,6 +7321,9 @@ kill-visual-line
 If ARG is zero, kill the text before point on the current visual
 line.
 
+If `kill-whole-line' is non-nil, and this command is invoked at
+start of a line that ands in a newline, kill the newline as well.
+
 If you want to append the killed line to the last killed text,
 use \\[append-next-kill] before \\[kill-line].
 
@@ -7331,18 +7336,26 @@ kill-visual-line
   ;; Like in `kill-line', it's better to move point to the other end
   ;; of the kill before killing.
   (let ((opoint (point))
-       (kill-whole-line (and kill-whole-line (bolp))))
+        (kill-whole-line (and kill-whole-line (bolp)))
+        (orig-y (cdr (nth 2 (posn-at-point)))))
     (if arg
        (vertical-motion (prefix-numeric-value arg))
       (end-of-visual-line 1)
       (if (= (point) opoint)
          (vertical-motion 1)
-       ;; Skip any trailing whitespace at the end of the visual line.
-       ;; We used to do this only if `show-trailing-whitespace' is
-       ;; nil, but that's wrong; the correct thing would be to check
-       ;; whether the trailing whitespace is highlighted.  But, it's
-       ;; OK to just do this unconditionally.
-       (skip-chars-forward " \t")))
+        ;; The first condition below verifies we are still on the same
+        ;; screen line, i.e. that the line isn't continued, and that
+        ;; end-of-visual-line didn't overshoot due to complications
+        ;; like display or overlay strings, intangible text, etc.:
+        ;; otherwise, we don't want to kill a character that's
+        ;; unrelated to the place where the visual line wrapped.
+        (and (= (cdr (nth 2 (posn-at-point))) orig-y)
+             ;; Make sure we delete the character where the line wraps
+             ;; under visual-line-mode.
+             (or (looking-at-p "[ \t]")
+                 (and word-wrap-by-category
+                      (aref (char-category-set (following-char)) ?\|)))
+             (forward-char))))
     (kill-region opoint (if (and kill-whole-line (= (following-char) ?\n))
                            (1+ (point))
                          (point)))))