bug#32744: There seems to be a bug in current-column in character mode

[Eli Zaretskii]

> From: Kalman Reti <kalman.reti@gmail.com>
> Date: Sun, 16 Sep 2018 19:44:33 -0400
> Cc: 32744@debbugs.gnu.org
> 
>  I get 8 in GUI frames as well.
> 
>  The display spec (space :width 7) means 7 times the width of the
>  default face's font.  So if your default GUI font is monospaced, you
>  should get 8 in both GUI and text-mode frames.
> 
>  So I'm unsure how come you get 2 in GUI mode.  Maybe you've omitted
>  some important detail in the reproducer.
> 
> I apologize; you are correct, it gets 8 in gui mode as well. I must have been 
> confused about which window
> was running which version of emacs while I was experimenting.
> 
> But is that the correct value? The bubbles game code clearly wants it to mean 
> 'the number of forward-chars I
> have to do from the beginning of the line to get here' not 'what column does 
> it visually look like I'm on'.
> 
> The doc string for the function doesn't clarify the intended meaning.
> 
> If you do m-x bubbles immediately followed by m-: (current-column) it returns 
> 1 in the gui case but a number
> larger than one (which depends upon the :width keyword of the wide-displaying 
> space), but for the game to
> work in the character case it needs to return 1.

It's a bug in bubbles.el, now fixed on the emacs-26 branch.  The patch
is below, so you can apply it locally if you want.

The patch does some cleanup, so it might not be easy to spot the
crucial problem it fixes.  The root cause of the bug was that
bubbles--initialize had 2 code paths in it: one for GUI frames (which
worked), and another for TTY frames (which didn't, and it seems to me
that no one has ever tried it).  The GUI part computed the offsets as
lists, i.e. '(N), whereas the TTY part produced just N, a simple
number.  And the display property 'space' treats each of these
differently: '(N) is treated as pixel width, while N is treated as a
multiple of the default character width (your example only used the N
form, so it produced 8 instead of 1 from current-column).  Once we
produce '(N) both in GUI and in TTY frames, the game works in both.

Here's the patch I promised; I'm closing the bug report.

diff --git a/lisp/play/bubbles.el b/lisp/play/bubbles.el
index e30838d..ee2135b 100644
--- a/lisp/play/bubbles.el
+++ b/lisp/play/bubbles.el
@@ -250,10 +250,10 @@ bubbles--images-ok
   "Indicate whether images have been created successfully.")
 
 (defvar bubbles--col-offset 0
-  "Horizontal offset for centering the bubbles grid.")
+  "Horizontal offset for centering the bubbles grid, in pixels.")
 
 (defvar bubbles--row-offset 0
-  "Vertical offset for centering the bubbles grid.")
+  "Vertical offset for centering the bubbles grid, in pixels.")
 
 (defvar bubbles--save-data nil
   "List containing bubbles save data (SCORE BUFFERCONTENTS).")
@@ -960,33 +960,26 @@ bubbles-quit
 (defun bubbles--compute-offsets ()
   "Update horizontal and vertical offsets for centering the bubbles grid.
 Set `bubbles--col-offset' and `bubbles--row-offset'."
-  (cond ((and (display-images-p)
-              bubbles--images-ok
-              (not (eq bubbles-graphics-theme 'ascii))
-              (fboundp 'window-inside-pixel-edges))
-         ;; compute offset in units of pixels
-         (let ((bubbles--image-size
-                (car (image-size (car bubbles--images) t))))
-           (setq bubbles--col-offset
-                 (list
-                  (max 0 (/ (- (nth 2 (window-inside-pixel-edges))
-                               (nth 0 (window-inside-pixel-edges))
-                               (* ( + bubbles--image-size 2) ;; margin
-                                  (bubbles--grid-width))) 2))))
-           (setq bubbles--row-offset
-                 (list
-                  (max 0 (/ (- (nth 3 (window-inside-pixel-edges))
-                               (nth 1 (window-inside-pixel-edges))
-                               (* (+ bubbles--image-size 1) ;; margin
-                                  (bubbles--grid-height))) 2))))))
-        (t
-         ;; compute offset in units of chars
-         (setq bubbles--col-offset
-               (max 0 (/ (- (window-width)
-                            (bubbles--grid-width)) 2)))
-         (setq bubbles--row-offset
-               (max 0 (/ (- (window-height)
-                            (bubbles--grid-height) 2) 2))))))
+  (let* ((use-images-p (and (display-images-p)
+                            bubbles--images-ok
+                            (not (eq bubbles-graphics-theme 'ascii))))
+         (bubbles--image-size
+          (if use-images-p (car (image-size (car bubbles--images) t)) 1))
+         ;; In GUI mode, leave thin margins around the images.
+         (image-hor-size
+          (if use-images-p (+ bubbles--image-size 2) bubbles--image-size))
+         (image-vert-size
+          (if use-images-p (1+ bubbles--image-size) bubbles--image-size)))
+    (setq bubbles--col-offset
+          (max 0 (/ (- (nth 2 (window-body-pixel-edges))
+                       (nth 0 (window-body-pixel-edges))
+                       (* image-hor-size (bubbles--grid-width)))
+                    2)))
+    (setq bubbles--row-offset
+          (max 0 (/ (- (nth 3 (window-body-pixel-edges))
+                       (nth 1 (window-body-pixel-edges))
+                       (* image-vert-size (bubbles--grid-height)))
+                    2)))))
 
 (defun bubbles--remove-overlays ()
   "Remove all overlays."
@@ -1007,7 +1000,8 @@ bubbles--initialize
     (insert " ")
     (put-text-property (point-min) (point)
                        'display
-                       (cons 'space (list :height bubbles--row-offset)))
+                       (cons 'space (list :height
+                                          (list bubbles--row-offset))))
     (insert "\n")
     (let ((max-char (length (bubbles--colors))))
       (dotimes (i (bubbles--grid-height))
@@ -1015,7 +1009,8 @@ bubbles--initialize
           (insert " ")
           (put-text-property p (point)
                              'display
-                             (cons 'space (list :width bubbles--col-offset))))
+                             (cons 'space (list :width
+                                                (list bubbles--col-offset)))))
         (dotimes (j (bubbles--grid-width))
           (let* ((index (random max-char))
                  (char (nth index bubbles-chars)))
@@ -1025,7 +1020,8 @@ bubbles--initialize
       (insert "\n ")
       (put-text-property (1- (point)) (point)
                          'display
-                         (cons 'space (list :width bubbles--col-offset))))
+                         (cons 'space (list :width
+                                            (list bubbles--col-offset)))))
     (put-text-property (point-min) (point-max) 'pointer 'arrow))
   (bubbles-mode)
   (bubbles--reset-score)
@@ -1177,7 +1173,7 @@ bubbles--show-scores
       (insert " ")
       (put-text-property (1- (point)) (point)
                          'display
-                         (cons 'space (list :width bubbles--col-offset)))
+                         (cons 'space (list :width (list 
bubbles--col-offset))))
       (insert (format "Score:    %4d" bubbles--score))
       (put-text-property pos (point) 'status t))))
 
@@ -1197,7 +1193,7 @@ bubbles--game-over
     (insert "\n ")
     (put-text-property (1- (point)) (point)
                        'display
-                       (cons 'space (list :width bubbles--col-offset)))
+                       (cons 'space (list :width (list bubbles--col-offset))))
     (insert "Game Over!"))
   ;; save score
   (gamegrid-add-score (format "bubbles-%s-%d-%d-%d-scores"