bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[angelo.graziosi]

Maybe this is not a bug but I want to flag for completeness..

The following should be reproducible with the following simple init.el:

$ cat init.el
;;
(global-hl-line-mode 1)

 
Now, in the scratch buffer, copy and paste many times its text (;; This 
buffer...) so that the scratch buffer has many lines, not all contained in the 
Emacs window/frame. Move the cursor on a line, more or less at the center of 
the window. The line is highlighted (in green, by default). Now move slowly 
up/down the scroll bar, then the highlighted line seems to flicker as if it 
were redrawn at (almost) each pixel movement of the scroll bar.

Is this to be expected? Other software with same feature do not show this 
"flickering" and the highlight moves synchronously with the text.

The effect is visible not only with vertical scroll bar but also with the 
horizontal.

I see this with my master builds on Windows, GNU/linux and OSX.


Angelo

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Eli Zaretskii]

> Date: Tue, 10 May 2016 21:31:26 +0200 (CEST)
> From: "angelo.graziosi@libero.it" <angelo.graziosi@libero.it>
> 
> (global-hl-line-mode 1)
> 
>  
> Now, in the scratch buffer, copy and paste many times its text (;; This 
> buffer...) so that the scratch buffer has many lines, not all contained in the 
> Emacs window/frame. Move the cursor on a line, more or less at the center of 
> the window. The line is highlighted (in green, by default). Now move slowly 
> up/down the scroll bar, then the highlighted line seems to flicker as if it 
> were redrawn at (almost) each pixel movement of the scroll bar.
> 
> Is this to be expected?

I think so.  HL-Line mode works by hooking both pre-command-hook and
post-command-hook; it removes its highlight overlay in the former and
reapplies it in the latter.  When you move the scroll-bar thumb, the
hooks are called several times (at least twice) per mouse drag event,
and since Emacs tracks the mouse moves at pixel granularity, you
usually have much more calls to the hooks.  The result is constant
redrawing of the highlighted line, and that causes flickering.

> Other software with same feature do not show this 
> "flickering" and the highlight moves synchronously with the text.

To reduce the number of redraws, hl-line.el should switch to a
different method of moving the overlay, I think.  I didn't try, so I
cannot be sure this is doable with the existing facilities, but
someone who is motivated enough should try using, e.g.,
pre-redisplay-function.

It could also be that hl-line should be smarter, and refrain from
removing and reapplying the overlay if point didn't move, or stays
within the same screen line.  Again, I didn't try that, so perhaps it
won't work for some reason.

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

On Wed, 11 May 2016 11:13:05 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> Date: Tue, 10 May 2016 21:31:26 +0200 (CEST)
>> From: "angelo.graziosi@libero.it" <angelo.graziosi@libero.it>
>> 
>> (global-hl-line-mode 1)
>> 
>>  
>> Now, in the scratch buffer, copy and paste many times its text (;; This 
>> buffer...) so that the scratch buffer has many lines, not all contained in the 
>> Emacs window/frame. Move the cursor on a line, more or less at the center of 
>> the window. The line is highlighted (in green, by default). Now move slowly 
>> up/down the scroll bar, then the highlighted line seems to flicker as if it 
>> were redrawn at (almost) each pixel movement of the scroll bar.
>> 
>> Is this to be expected?
>
> I think so.  HL-Line mode works by hooking both pre-command-hook and
> post-command-hook; it removes its highlight overlay in the former and
> reapplies it in the latter.  When you move the scroll-bar thumb, the
> hooks are called several times (at least twice) per mouse drag event,
> and since Emacs tracks the mouse moves at pixel granularity, you
> usually have much more calls to the hooks.  The result is constant
> redrawing of the highlighted line, and that causes flickering.
>
>> Other software with same feature do not show this 
>> "flickering" and the highlight moves synchronously with the text.
>
> To reduce the number of redraws, hl-line.el should switch to a
> different method of moving the overlay, I think.  I didn't try, so I
> cannot be sure this is doable with the existing facilities, but
> someone who is motivated enough should try using, e.g.,
> pre-redisplay-function.
>
> It could also be that hl-line should be smarter, and refrain from
> removing and reapplying the overlay if point didn't move, or stays
> within the same screen line.  Again, I didn't try that, so perhaps it
> won't work for some reason.

What you say about the use of pre- and post-command-hook and moving
overlays may well be (perhaps the main) part of the problem here, but I
found out that there's an specific condition inducing it: I noticed that
the flickering only happens when global-hl-line-mode is enabled, but not
when hl-line-mode is enabled, and it turns out that the crucial
difference between them is that in the former,
global-hl-line-sticky-flag is nil by default, while in the latter,
hl-line-sticky-flag is t by default: when I set
global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
flickering no longer happens, and when I set hl-line-sticky-flag to nil
and then enable hl-line-mode, the flickering does happen.

Steve Berman

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: "angelo.graziosi\@libero.it" <angelo.graziosi@libero.it>,  23510@debbugs.gnu.org
> Date: Wed, 11 May 2016 11:25:24 +0200
> 
> What you say about the use of pre- and post-command-hook and moving
> overlays may well be (perhaps the main) part of the problem here, but I
> found out that there's an specific condition inducing it: I noticed that
> the flickering only happens when global-hl-line-mode is enabled, but not
> when hl-line-mode is enabled, and it turns out that the crucial
> difference between them is that in the former,
> global-hl-line-sticky-flag is nil by default, while in the latter,
> hl-line-sticky-flag is t by default: when I set
> global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
> flickering no longer happens, and when I set hl-line-sticky-flag to nil
> and then enable hl-line-mode, the flickering does happen.

If you can suggest a patch that fixes that and doesn't have any
adverse effects, please do.

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

[Resending, since I sent this last night but it apparently never
arrived; if it took a wrong turn and arrives later, sorry for the
duplication.]

On Wed, 11 May 2016 12:44:43 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: "angelo.graziosi\@libero.it" <angelo.graziosi@libero.it>,
>> 23510@debbugs.gnu.org
>> Date: Wed, 11 May 2016 11:25:24 +0200
>> 
>> What you say about the use of pre- and post-command-hook and moving
>> overlays may well be (perhaps the main) part of the problem here, but I
>> found out that there's an specific condition inducing it: I noticed that
>> the flickering only happens when global-hl-line-mode is enabled, but not
>> when hl-line-mode is enabled, and it turns out that the crucial
>> difference between them is that in the former,
>> global-hl-line-sticky-flag is nil by default, while in the latter,
>> hl-line-sticky-flag is t by default: when I set
>> global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
>> flickering no longer happens, and when I set hl-line-sticky-flag to nil
>> and then enable hl-line-mode, the flickering does happen.
>
> If you can suggest a patch that fixes that and doesn't have any
> adverse effects, please do.

Ok, I've given it a try; patch below.  Debugging showed that the
flickering is caused by calling delete-overlay on pre-command-hook, so I
eliminated the uses of this hook and replaced them with the new
functions hl-line-maybe-unhighlight and global-hl-line-maybe-unhighlight
added to post-command-hook that call delete-overlay under more specific
conditions.  According to my tests this does not lead to flickering and
highlights and unhighlights as before (but others should test the code,
in case I missed or messed up anything).

I also noticed that the definition of global-hl-line-mode fails to
remove global-hl-line-highlight from change-major-mode-hook when the
mode is disabled; I assume this was an oversite, so I rectified it.

If my fix is found acceptable, I assume it should go to master, since
the flickering problem was probably always there, so is not a regression
in emacs-25.  With that in mind, I took the opportunity to make
global-hl-line-mode automatically taken effect in all live windows when
global-hl-line-sticky-flag is non-nil (the new function
global-hl-line-highlight-all); in the current (and I assume original)
implementation, enabling global-hl-line-mode only highlights the current
line of the selected window, even when global-hl-line-sticky-flag is
non-nil (the highlighting appears in other windows when they become
selected).  This change seems consistent with the idea of Global-Hl-Line
mode, expressed in its doc string ("Toggle line highlighting in all
buffers").  I tested it with three windows in each of two frames (the
most that comfortably fit on my display), and the highlighting was as
fast as with two windows on one frame, but I suppose it's possible that
more windows on more frames might be slower.

Steve Berman

diff --git a/lisp/hl-line.el b/lisp/hl-line.el
index 25c8a08..642a8ba 100644
--- a/lisp/hl-line.el
+++ b/lisp/hl-line.el
@@ -126,6 +126,9 @@ hl-line-range-function
 
 This variable is expected to be made buffer-local by modes.")
 
+(defvar hl-line-overlay-buffer nil
+  "Most recently visited buffer in which Hl-Line mode is enabled.")
+
 ;;;###autoload
 (define-minor-mode hl-line-mode
   "Toggle highlighting of the current line (Hl-Line mode).
@@ -142,22 +145,21 @@ hl-line-mode
 
 When `hl-line-sticky-flag' is nil, Hl-Line mode highlights the
 line about point in the selected window only.  In this case, it
-uses the function `hl-line-unhighlight' on `pre-command-hook' in
+uses the function `hl-line-maybe-unhighlight' in
 addition to `hl-line-highlight' on `post-command-hook'."
   :group 'hl-line
   (if hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'hl-line-unhighlight nil t)
-        (if hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'hl-line-unhighlight t)
-          (add-hook 'pre-command-hook #'hl-line-unhighlight nil t))
         (hl-line-highlight)
-	(add-hook 'post-command-hook #'hl-line-highlight nil t))
+        (setq hl-line-overlay-buffer (current-buffer))
+	(add-hook 'post-command-hook #'hl-line-highlight nil t)
+        (add-hook 'post-command-hook #'hl-line-maybe-unhighlight))
     (remove-hook 'post-command-hook #'hl-line-highlight t)
     (hl-line-unhighlight)
     (remove-hook 'change-major-mode-hook #'hl-line-unhighlight t)
-    (remove-hook 'pre-command-hook #'hl-line-unhighlight t)))
+    (remove-hook 'post-command-hook #'hl-line-maybe-unhighlight t)))
 
 (defun hl-line-make-overlay ()
   (let ((ol (make-overlay (point) (point))))
@@ -181,6 +183,22 @@ hl-line-unhighlight
   (when hl-line-overlay
     (delete-overlay hl-line-overlay)))
 
+(defun hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Hl-Line overlay on the current line.
+Specifically, when `hl-line-sticky-flag' is nil deactivate all
+such overlays in all buffers except the current one."
+  (let ((hlob hl-line-overlay-buffer)
+        (curbuf (current-buffer)))
+    (when (and (not hl-line-sticky-flag)
+               (not (eq curbuf hlob))
+               (not (minibufferp)))
+      (with-current-buffer hlob
+        (when (overlayp hl-line-overlay)
+          (delete-overlay hl-line-overlay))))
+    (when (and (overlayp hl-line-overlay)
+               (eq (overlay-buffer hl-line-overlay) curbuf))
+      (setq hl-line-overlay-buffer curbuf))))
+
 ;;;###autoload
 (define-minor-mode global-hl-line-mode
   "Toggle line highlighting in all buffers (Global Hl-Line mode).
@@ -189,25 +207,24 @@ global-hl-line-mode
 the mode if ARG is omitted or nil.
 
 If `global-hl-line-sticky-flag' is non-nil, Global Hl-Line mode
-highlights the line about the current buffer's point in all
+highlights the line about the current buffer's point in all live
 windows.
 
-Global-Hl-Line mode uses the functions `global-hl-line-unhighlight' and
-`global-hl-line-highlight' on `pre-command-hook' and `post-command-hook'."
+Global-Hl-Line mode uses the functions `global-hl-line-highlight'
+and `global-hl-line-maybe-unhighlight' on `post-command-hook'."
   :global t
   :group 'hl-line
   (if global-hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
-        (if global-hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-          (add-hook 'pre-command-hook #'global-hl-line-unhighlight))
-        (global-hl-line-highlight)
-	(add-hook 'post-command-hook #'global-hl-line-highlight))
+        (global-hl-line-highlight-all)
+	(add-hook 'post-command-hook #'global-hl-line-highlight)
+        (add-hook 'post-command-hook #'global-hl-line-maybe-unhighlight))
     (global-hl-line-unhighlight-all)
-    (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-    (remove-hook 'post-command-hook #'global-hl-line-highlight)))
+    (remove-hook 'post-command-hook #'global-hl-line-highlight)
+    (remove-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
+    (remove-hook 'post-command-hook #'global-hl-line-maybe-unhighlight)))
 
 (defun global-hl-line-highlight ()
   "Highlight the current line in the current window."
@@ -222,11 +239,33 @@ global-hl-line-highlight
 		     (selected-window)))
       (hl-line-move global-hl-line-overlay))))
 
+(defun global-hl-line-highlight-all ()
+  "Highlight the current line in all live windows."
+  (walk-windows (lambda (w)
+                  (with-current-buffer (window-buffer w)
+                    (global-hl-line-highlight)))
+                nil t))
+
 (defun global-hl-line-unhighlight ()
   "Deactivate the Global-Hl-Line overlay on the current line."
   (when global-hl-line-overlay
     (delete-overlay global-hl-line-overlay)))
 
+(defun global-hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Global-Hl-Line overlay on the current line.
+Specifically, when `global-hl-line-sticky-flag' is nil deactivate
+all such overlays in all buffers except the current one."
+  (mapc (lambda (ov)
+	  (let ((ovb (overlay-buffer ov)))
+            (when (and (not global-hl-line-sticky-flag)
+                       (bufferp ovb)
+                       (not (eq ovb (current-buffer)))
+                       (not (minibufferp)))
+		(with-current-buffer ovb
+                  (when (overlayp global-hl-line-overlay)
+                    (delete-overlay global-hl-line-overlay))))))
+        global-hl-line-overlays))
+
 (defun global-hl-line-unhighlight-all ()
   "Deactivate all Global-Hl-Line overlays."
   (mapc (lambda (ov)

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

On Wed, 11 May 2016 12:44:43 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: "angelo.graziosi\@libero.it" <angelo.graziosi@libero.it>,
>> 23510@debbugs.gnu.org
>> Date: Wed, 11 May 2016 11:25:24 +0200
>> 
>> What you say about the use of pre- and post-command-hook and moving
>> overlays may well be (perhaps the main) part of the problem here, but I
>> found out that there's an specific condition inducing it: I noticed that
>> the flickering only happens when global-hl-line-mode is enabled, but not
>> when hl-line-mode is enabled, and it turns out that the crucial
>> difference between them is that in the former,
>> global-hl-line-sticky-flag is nil by default, while in the latter,
>> hl-line-sticky-flag is t by default: when I set
>> global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
>> flickering no longer happens, and when I set hl-line-sticky-flag to nil
>> and then enable hl-line-mode, the flickering does happen.
>
> If you can suggest a patch that fixes that and doesn't have any
> adverse effects, please do.

Ok, I've given it a try; patch below.  Debugging showed that the
flickering is caused by calling delete-overlay on pre-command-hook, so I
eliminated the uses of this hook and replaced them with the new
functions hl-line-maybe-unhighlight and global-hl-line-maybe-unhighlight
added to post-command-hook that call delete-overlay under more specific
conditions.  According to my tests this does not lead to flickering and
highlights and unhighlights as before (but others should test the code,
in case I missed or messed up anything).

I also noticed that the definition of global-hl-line-mode fails to
remove global-hl-line-highlight from change-major-mode-hook when the
mode is disabled; I assume this was an oversite, so I rectified it.

If my fix is found acceptable, I assume it should go to master, since
the flickering problem was probably always there, so is not a regression
in emacs-25.  With that in mind, I took the opportunity to make
global-hl-line-mode automatically taken effect in all live windows when
global-hl-line-sticky-flag is non-nil (the new function
global-hl-line-highlight-all); in the current (and I assume original)
implementation, enabling global-hl-line-mode only highlights the current
line of the selected window, even when global-hl-line-sticky-flag is
non-nil (the highlighting appears in other windows when they become
selected).  This change seems consistent with the idea of Global-Hl-Line
mode, expressed in its doc string ("Toggle line highlighting in all
buffers").  I tested it with three windows in each of two frames (the
most that comfortably fit on my display), and the highlighting was as
fast as with two windows on one frame, but I suppose it's possible that
more windows on more frames might be slower.

Steve Berman

diff --git a/lisp/hl-line.el b/lisp/hl-line.el
index 25c8a08..642a8ba 100644
--- a/lisp/hl-line.el
+++ b/lisp/hl-line.el
@@ -126,6 +126,9 @@ hl-line-range-function
 
 This variable is expected to be made buffer-local by modes.")
 
+(defvar hl-line-overlay-buffer nil
+  "Most recently visited buffer in which Hl-Line mode is enabled.")
+
 ;;;###autoload
 (define-minor-mode hl-line-mode
   "Toggle highlighting of the current line (Hl-Line mode).
@@ -142,22 +145,21 @@ hl-line-mode
 
 When `hl-line-sticky-flag' is nil, Hl-Line mode highlights the
 line about point in the selected window only.  In this case, it
-uses the function `hl-line-unhighlight' on `pre-command-hook' in
+uses the function `hl-line-maybe-unhighlight' in
 addition to `hl-line-highlight' on `post-command-hook'."
   :group 'hl-line
   (if hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'hl-line-unhighlight nil t)
-        (if hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'hl-line-unhighlight t)
-          (add-hook 'pre-command-hook #'hl-line-unhighlight nil t))
         (hl-line-highlight)
-	(add-hook 'post-command-hook #'hl-line-highlight nil t))
+        (setq hl-line-overlay-buffer (current-buffer))
+	(add-hook 'post-command-hook #'hl-line-highlight nil t)
+        (add-hook 'post-command-hook #'hl-line-maybe-unhighlight))
     (remove-hook 'post-command-hook #'hl-line-highlight t)
     (hl-line-unhighlight)
     (remove-hook 'change-major-mode-hook #'hl-line-unhighlight t)
-    (remove-hook 'pre-command-hook #'hl-line-unhighlight t)))
+    (remove-hook 'post-command-hook #'hl-line-maybe-unhighlight t)))
 
 (defun hl-line-make-overlay ()
   (let ((ol (make-overlay (point) (point))))
@@ -181,6 +183,22 @@ hl-line-unhighlight
   (when hl-line-overlay
     (delete-overlay hl-line-overlay)))
 
+(defun hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Hl-Line overlay on the current line.
+Specifically, when `hl-line-sticky-flag' is nil deactivate all
+such overlays in all buffers except the current one."
+  (let ((hlob hl-line-overlay-buffer)
+        (curbuf (current-buffer)))
+    (when (and (not hl-line-sticky-flag)
+               (not (eq curbuf hlob))
+               (not (minibufferp)))
+      (with-current-buffer hlob
+        (when (overlayp hl-line-overlay)
+          (delete-overlay hl-line-overlay))))
+    (when (and (overlayp hl-line-overlay)
+               (eq (overlay-buffer hl-line-overlay) curbuf))
+      (setq hl-line-overlay-buffer curbuf))))
+
 ;;;###autoload
 (define-minor-mode global-hl-line-mode
   "Toggle line highlighting in all buffers (Global Hl-Line mode).
@@ -189,25 +207,24 @@ global-hl-line-mode
 the mode if ARG is omitted or nil.
 
 If `global-hl-line-sticky-flag' is non-nil, Global Hl-Line mode
-highlights the line about the current buffer's point in all
+highlights the line about the current buffer's point in all live
 windows.
 
-Global-Hl-Line mode uses the functions `global-hl-line-unhighlight' and
-`global-hl-line-highlight' on `pre-command-hook' and `post-command-hook'."
+Global-Hl-Line mode uses the functions `global-hl-line-highlight'
+and `global-hl-line-maybe-unhighlight' on `post-command-hook'."
   :global t
   :group 'hl-line
   (if global-hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
-        (if global-hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-          (add-hook 'pre-command-hook #'global-hl-line-unhighlight))
-        (global-hl-line-highlight)
-	(add-hook 'post-command-hook #'global-hl-line-highlight))
+        (global-hl-line-highlight-all)
+	(add-hook 'post-command-hook #'global-hl-line-highlight)
+        (add-hook 'post-command-hook #'global-hl-line-maybe-unhighlight))
     (global-hl-line-unhighlight-all)
-    (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-    (remove-hook 'post-command-hook #'global-hl-line-highlight)))
+    (remove-hook 'post-command-hook #'global-hl-line-highlight)
+    (remove-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
+    (remove-hook 'post-command-hook #'global-hl-line-maybe-unhighlight)))
 
 (defun global-hl-line-highlight ()
   "Highlight the current line in the current window."
@@ -222,11 +239,33 @@ global-hl-line-highlight
 		     (selected-window)))
       (hl-line-move global-hl-line-overlay))))
 
+(defun global-hl-line-highlight-all ()
+  "Highlight the current line in all live windows."
+  (walk-windows (lambda (w)
+                  (with-current-buffer (window-buffer w)
+                    (global-hl-line-highlight)))
+                nil t))
+
 (defun global-hl-line-unhighlight ()
   "Deactivate the Global-Hl-Line overlay on the current line."
   (when global-hl-line-overlay
     (delete-overlay global-hl-line-overlay)))
 
+(defun global-hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Global-Hl-Line overlay on the current line.
+Specifically, when `global-hl-line-sticky-flag' is nil deactivate
+all such overlays in all buffers except the current one."
+  (mapc (lambda (ov)
+	  (let ((ovb (overlay-buffer ov)))
+            (when (and (not global-hl-line-sticky-flag)
+                       (bufferp ovb)
+                       (not (eq ovb (current-buffer)))
+                       (not (minibufferp)))
+		(with-current-buffer ovb
+                  (when (overlayp global-hl-line-overlay)
+                    (delete-overlay global-hl-line-overlay))))))
+        global-hl-line-overlays))
+
 (defun global-hl-line-unhighlight-all ()
   "Deactivate all Global-Hl-Line overlays."
   (mapc (lambda (ov)

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

[I sent this last night, but it never arrived, so I resent it about an
hour ago, but that also hasn't arrived, so now I'm sending again from a
different email address.]

On Wed, 11 May 2016 12:44:43 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: "angelo.graziosi\@libero.it" <angelo.graziosi@libero.it>,
>> 23510@debbugs.gnu.org
>> Date: Wed, 11 May 2016 11:25:24 +0200
>> 
>> What you say about the use of pre- and post-command-hook and moving
>> overlays may well be (perhaps the main) part of the problem here, but I
>> found out that there's an specific condition inducing it: I noticed that
>> the flickering only happens when global-hl-line-mode is enabled, but not
>> when hl-line-mode is enabled, and it turns out that the crucial
>> difference between them is that in the former,
>> global-hl-line-sticky-flag is nil by default, while in the latter,
>> hl-line-sticky-flag is t by default: when I set
>> global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
>> flickering no longer happens, and when I set hl-line-sticky-flag to nil
>> and then enable hl-line-mode, the flickering does happen.
>
> If you can suggest a patch that fixes that and doesn't have any
> adverse effects, please do.

Ok, I've given it a try; patch below.  Debugging showed that the
flickering is caused by calling delete-overlay on pre-command-hook, so I
eliminated the uses of this hook and replaced them with the new
functions hl-line-maybe-unhighlight and global-hl-line-maybe-unhighlight
added to post-command-hook that call delete-overlay under more specific
conditions.  According to my tests this does not lead to flickering and
highlights and unhighlights as before (but others should test the code,
in case I missed or messed up anything).

I also noticed that the definition of global-hl-line-mode fails to
remove global-hl-line-highlight from change-major-mode-hook when the
mode is disabled; I assume this was an oversite, so I rectified it.

If my fix is found acceptable, I assume it should go to master, since
the flickering problem was probably always there, so is not a regression
in emacs-25.  With that in mind, I took the opportunity to make
global-hl-line-mode automatically taken effect in all live windows when
global-hl-line-sticky-flag is non-nil (the new function
global-hl-line-highlight-all); in the current (and I assume original)
implementation, enabling global-hl-line-mode only highlights the current
line of the selected window, even when global-hl-line-sticky-flag is
non-nil (the highlighting appears in other windows when they become
selected).  This change seems consistent with the idea of Global-Hl-Line
mode, expressed in its doc string ("Toggle line highlighting in all
buffers").  I tested it with three windows in each of two frames (the
most that comfortably fit on my display), and the highlighting was as
fast as with two windows on one frame, but I suppose it's possible that
more windows on more frames might be slower.

Steve Berman

diff --git a/lisp/hl-line.el b/lisp/hl-line.el
index 25c8a08..642a8ba 100644
--- a/lisp/hl-line.el
+++ b/lisp/hl-line.el
@@ -126,6 +126,9 @@ hl-line-range-function
 
 This variable is expected to be made buffer-local by modes.")
 
+(defvar hl-line-overlay-buffer nil
+  "Most recently visited buffer in which Hl-Line mode is enabled.")
+
 ;;;###autoload
 (define-minor-mode hl-line-mode
   "Toggle highlighting of the current line (Hl-Line mode).
@@ -142,22 +145,21 @@ hl-line-mode
 
 When `hl-line-sticky-flag' is nil, Hl-Line mode highlights the
 line about point in the selected window only.  In this case, it
-uses the function `hl-line-unhighlight' on `pre-command-hook' in
+uses the function `hl-line-maybe-unhighlight' in
 addition to `hl-line-highlight' on `post-command-hook'."
   :group 'hl-line
   (if hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'hl-line-unhighlight nil t)
-        (if hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'hl-line-unhighlight t)
-          (add-hook 'pre-command-hook #'hl-line-unhighlight nil t))
         (hl-line-highlight)
-	(add-hook 'post-command-hook #'hl-line-highlight nil t))
+        (setq hl-line-overlay-buffer (current-buffer))
+	(add-hook 'post-command-hook #'hl-line-highlight nil t)
+        (add-hook 'post-command-hook #'hl-line-maybe-unhighlight))
     (remove-hook 'post-command-hook #'hl-line-highlight t)
     (hl-line-unhighlight)
     (remove-hook 'change-major-mode-hook #'hl-line-unhighlight t)
-    (remove-hook 'pre-command-hook #'hl-line-unhighlight t)))
+    (remove-hook 'post-command-hook #'hl-line-maybe-unhighlight t)))
 
 (defun hl-line-make-overlay ()
   (let ((ol (make-overlay (point) (point))))
@@ -181,6 +183,22 @@ hl-line-unhighlight
   (when hl-line-overlay
     (delete-overlay hl-line-overlay)))
 
+(defun hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Hl-Line overlay on the current line.
+Specifically, when `hl-line-sticky-flag' is nil deactivate all
+such overlays in all buffers except the current one."
+  (let ((hlob hl-line-overlay-buffer)
+        (curbuf (current-buffer)))
+    (when (and (not hl-line-sticky-flag)
+               (not (eq curbuf hlob))
+               (not (minibufferp)))
+      (with-current-buffer hlob
+        (when (overlayp hl-line-overlay)
+          (delete-overlay hl-line-overlay))))
+    (when (and (overlayp hl-line-overlay)
+               (eq (overlay-buffer hl-line-overlay) curbuf))
+      (setq hl-line-overlay-buffer curbuf))))
+
 ;;;###autoload
 (define-minor-mode global-hl-line-mode
   "Toggle line highlighting in all buffers (Global Hl-Line mode).
@@ -189,25 +207,24 @@ global-hl-line-mode
 the mode if ARG is omitted or nil.
 
 If `global-hl-line-sticky-flag' is non-nil, Global Hl-Line mode
-highlights the line about the current buffer's point in all
+highlights the line about the current buffer's point in all live
 windows.
 
-Global-Hl-Line mode uses the functions `global-hl-line-unhighlight' and
-`global-hl-line-highlight' on `pre-command-hook' and `post-command-hook'."
+Global-Hl-Line mode uses the functions `global-hl-line-highlight'
+and `global-hl-line-maybe-unhighlight' on `post-command-hook'."
   :global t
   :group 'hl-line
   (if global-hl-line-mode
       (progn
         ;; In case `kill-all-local-variables' is called.
         (add-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
-        (if global-hl-line-sticky-flag
-            (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-          (add-hook 'pre-command-hook #'global-hl-line-unhighlight))
-        (global-hl-line-highlight)
-	(add-hook 'post-command-hook #'global-hl-line-highlight))
+        (global-hl-line-highlight-all)
+	(add-hook 'post-command-hook #'global-hl-line-highlight)
+        (add-hook 'post-command-hook #'global-hl-line-maybe-unhighlight))
     (global-hl-line-unhighlight-all)
-    (remove-hook 'pre-command-hook #'global-hl-line-unhighlight)
-    (remove-hook 'post-command-hook #'global-hl-line-highlight)))
+    (remove-hook 'post-command-hook #'global-hl-line-highlight)
+    (remove-hook 'change-major-mode-hook #'global-hl-line-unhighlight)
+    (remove-hook 'post-command-hook #'global-hl-line-maybe-unhighlight)))
 
 (defun global-hl-line-highlight ()
   "Highlight the current line in the current window."
@@ -222,11 +239,33 @@ global-hl-line-highlight
 		     (selected-window)))
       (hl-line-move global-hl-line-overlay))))
 
+(defun global-hl-line-highlight-all ()
+  "Highlight the current line in all live windows."
+  (walk-windows (lambda (w)
+                  (with-current-buffer (window-buffer w)
+                    (global-hl-line-highlight)))
+                nil t))
+
 (defun global-hl-line-unhighlight ()
   "Deactivate the Global-Hl-Line overlay on the current line."
   (when global-hl-line-overlay
     (delete-overlay global-hl-line-overlay)))
 
+(defun global-hl-line-maybe-unhighlight ()
+  "Maybe deactivate the Global-Hl-Line overlay on the current line.
+Specifically, when `global-hl-line-sticky-flag' is nil deactivate
+all such overlays in all buffers except the current one."
+  (mapc (lambda (ov)
+	  (let ((ovb (overlay-buffer ov)))
+            (when (and (not global-hl-line-sticky-flag)
+                       (bufferp ovb)
+                       (not (eq ovb (current-buffer)))
+                       (not (minibufferp)))
+		(with-current-buffer ovb
+                  (when (overlayp global-hl-line-overlay)
+                    (delete-overlay global-hl-line-overlay))))))
+        global-hl-line-overlays))
+
 (defun global-hl-line-unhighlight-all ()
   "Deactivate all Global-Hl-Line overlays."
   (mapc (lambda (ov)

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

On Wed, 11 May 2016 12:44:43 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: "angelo.graziosi\@libero.it" <angelo.graziosi@libero.it>,
>> 23510@debbugs.gnu.org
>> Date: Wed, 11 May 2016 11:25:24 +0200
>> 
>> What you say about the use of pre- and post-command-hook and moving
>> overlays may well be (perhaps the main) part of the problem here, but I
>> found out that there's an specific condition inducing it: I noticed that
>> the flickering only happens when global-hl-line-mode is enabled, but not
>> when hl-line-mode is enabled, and it turns out that the crucial
>> difference between them is that in the former,
>> global-hl-line-sticky-flag is nil by default, while in the latter,
>> hl-line-sticky-flag is t by default: when I set
>> global-hl-line-sticky-flag to t and then enable global-hl-line-mode, the
>> flickering no longer happens, and when I set hl-line-sticky-flag to nil
>> and then enable hl-line-mode, the flickering does happen.
>
> If you can suggest a patch that fixes that and doesn't have any
> adverse effects, please do.

I have a patch, which I sent last night but it never arrived; I resent
it twice today, once from the same email address and once from a
different one, but neither of those have arrived, although other posts
have appeared in the list since my attempts were sent.  So this is just
a test to see if I can get through at all; if so, I'll try to see if I
can send the patch another way.

Steve Berman

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Angelo Graziosi]

> If my fix is found acceptable

I have tested the patch with this init.el file:

;;
(global-hl-line-mode 1)


and it seems to work..

Any other configuration for testing?

Ciao,
Angelo.

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

On Sat, 14 May 2016 09:42:13 +0200 Angelo Graziosi <angelo.graziosi@alice.it> wrote:

>> If my fix is found acceptable
>
> I have tested the patch with this init.el file:
>
> ;;
> (global-hl-line-mode 1)
>
>
> and it seems to work..

I take it you mean you see no flickering?  That would confirm the patch
fixes problem you reported.  You should also see, when you make the
frame have two (or more) windows, that the current line of only the
selected window is highlighted; this is the default behavior without the
patch and should obtain with the patch as well.

> Any other configuration for testing?

The default for global-hl-line-mode, which you tested with the above, is
for the value of global-hl-line-sticky-flag to be nil; if you set it to
non-nil, then when you make the frame display two (or more) windows, the
current line is highlighted in all of them.  A difference between the
behavior of the existing version of hl-line.el and the behavior with my
patch is that, in the former, the current line of each window displaying
a different buffer becomes highlighted only as soon as that window is
selected (and remains highlighted when another window is selected),
while with my patch, the current line in all live (visible) windows is
highlighted at once.  So this is a new feature and the question is, do
you prefer this behavior to the original behavior?  (If all the windows
display the same buffer, then with non-nil global-hl-line-sticky-flag
the highlighting appears in all of them simultaneously both with and
without the patch.)

It would be helpful if you'd also test the behavior of the patch with
the buffer local hl-line-mode.  Here, the default is for the value of
hl-line-sticky-flag to be non-nil, which means that if the same buffer
is displayed in two (or more) windows of a frame, the current line is
highlighted in each of these windows, and in addtion, when you select a
window displaying a different buffer, the line in the previously
selected window (displaying the buffer in which hl-line-mode was
enabled) remains highlighted.  If you set hl-line-sticky-flag to nil,
then only the current line of the selected window is highlighted.  These
behaviors of hl-line-mode should still be the same with the patch.  But
without the patch, setting hl-line-sticky-flag to nil results in
flickering of the highlighted line when moving the scroll bar with the
mouse; with the patch, there should be no flickering.

Steve Berman

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Angelo Graziosi]

Stephen,

I tried your patch without an init file and calling "M-x hl-line-mode" 
and then "M-x global-hl-line-mode" and I think I can reproduce all your 
arguments below.

Now my init file contains

(setq global-hl-line-sticky-flag t)
(global-hl-line-mode 1)

so that I can see the line in all windows of the frame.

Obviously, no flickering in all case you describe below..


  Angelo

Il 16/05/2016 23:33, Stephen Berman ha scritto:
> On Sat, 14 May 2016 09:42:13 +0200 Angelo Graziosi <angelo.graziosi@alice.it> wrote:
>
>>> If my fix is found acceptable
>>
>> I have tested the patch with this init.el file:
>>
>> ;;
>> (global-hl-line-mode 1)
>>
>>
>> and it seems to work..
>
> I take it you mean you see no flickering?  That would confirm the patch
> fixes problem you reported.  You should also see, when you make the
> frame have two (or more) windows, that the current line of only the
> selected window is highlighted; this is the default behavior without the
> patch and should obtain with the patch as well.
>
>> Any other configuration for testing?
>
> The default for global-hl-line-mode, which you tested with the above, is
> for the value of global-hl-line-sticky-flag to be nil; if you set it to
> non-nil, then when you make the frame display two (or more) windows, the
> current line is highlighted in all of them.  A difference between the
> behavior of the existing version of hl-line.el and the behavior with my
> patch is that, in the former, the current line of each window displaying
> a different buffer becomes highlighted only as soon as that window is
> selected (and remains highlighted when another window is selected),
> while with my patch, the current line in all live (visible) windows is
> highlighted at once.  So this is a new feature and the question is, do
> you prefer this behavior to the original behavior?  (If all the windows
> display the same buffer, then with non-nil global-hl-line-sticky-flag
> the highlighting appears in all of them simultaneously both with and
> without the patch.)
>
> It would be helpful if you'd also test the behavior of the patch with
> the buffer local hl-line-mode.  Here, the default is for the value of
> hl-line-sticky-flag to be non-nil, which means that if the same buffer
> is displayed in two (or more) windows of a frame, the current line is
> highlighted in each of these windows, and in addtion, when you select a
> window displaying a different buffer, the line in the previously
> selected window (displaying the buffer in which hl-line-mode was
> enabled) remains highlighted.  If you set hl-line-sticky-flag to nil,
> then only the current line of the selected window is highlighted.  These
> behaviors of hl-line-mode should still be the same with the patch.  But
> without the patch, setting hl-line-sticky-flag to nil results in
> flickering of the highlighted line when moving the scroll bar with the
> mouse; with the patch, there should be no flickering.
>
> Steve Berman
>

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Angelo Graziosi]

Any news about this issue? I am still using an Emacs build with 
Stephen's patch and I see no problem...

   Angelo

Il 17/05/2016 00:28, Angelo Graziosi ha scritto:
> Stephen,
>
> I tried your patch without an init file and calling "M-x hl-line-mode"
> and then "M-x global-hl-line-mode" and I think I can reproduce all your
> arguments below.
>
> Now my init file contains
>
> (setq global-hl-line-sticky-flag t)
> (global-hl-line-mode 1)
>
> so that I can see the line in all windows of the frame.
>
> Obviously, no flickering in all case you describe below..
>
>
>  Angelo
>
> Il 16/05/2016 23:33, Stephen Berman ha scritto:
>> On Sat, 14 May 2016 09:42:13 +0200 Angelo Graziosi
>> <angelo.graziosi@alice.it> wrote:
>>
>>>> If my fix is found acceptable
>>>
>>> I have tested the patch with this init.el file:
>>>
>>> ;;
>>> (global-hl-line-mode 1)
>>>
>>>
>>> and it seems to work..
>>
>> I take it you mean you see no flickering?  That would confirm the patch
>> fixes problem you reported.  You should also see, when you make the
>> frame have two (or more) windows, that the current line of only the
>> selected window is highlighted; this is the default behavior without the
>> patch and should obtain with the patch as well.
>>
>>> Any other configuration for testing?
>>
>> The default for global-hl-line-mode, which you tested with the above, is
>> for the value of global-hl-line-sticky-flag to be nil; if you set it to
>> non-nil, then when you make the frame display two (or more) windows, the
>> current line is highlighted in all of them.  A difference between the
>> behavior of the existing version of hl-line.el and the behavior with my
>> patch is that, in the former, the current line of each window displaying
>> a different buffer becomes highlighted only as soon as that window is
>> selected (and remains highlighted when another window is selected),
>> while with my patch, the current line in all live (visible) windows is
>> highlighted at once.  So this is a new feature and the question is, do
>> you prefer this behavior to the original behavior?  (If all the windows
>> display the same buffer, then with non-nil global-hl-line-sticky-flag
>> the highlighting appears in all of them simultaneously both with and
>> without the patch.)
>>
>> It would be helpful if you'd also test the behavior of the patch with
>> the buffer local hl-line-mode.  Here, the default is for the value of
>> hl-line-sticky-flag to be non-nil, which means that if the same buffer
>> is displayed in two (or more) windows of a frame, the current line is
>> highlighted in each of these windows, and in addtion, when you select a
>> window displaying a different buffer, the line in the previously
>> selected window (displaying the buffer in which hl-line-mode was
>> enabled) remains highlighted.  If you set hl-line-sticky-flag to nil,
>> then only the current line of the selected window is highlighted.  These
>> behaviors of hl-line-mode should still be the same with the patch.  But
>> without the patch, setting hl-line-sticky-flag to nil results in
>> flickering of the highlighted line when moving the scroll bar with the
>> mouse; with the patch, there should be no flickering.
>>
>> Steve Berman
>>

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Eli Zaretskii]

> Cc: 23510@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
> From: Angelo Graziosi <angelo.graziosi@alice.it>
> Date: Thu, 26 May 2016 00:13:28 +0200
> 
> Any news about this issue? I am still using an Emacs build with 
> Stephen's patch and I see no problem...

Would someone please install that?

bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

[Stephen Berman]

On Thu, 26 May 2016 05:44:48 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> Cc: 23510@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
>> From: Angelo Graziosi <angelo.graziosi@alice.it>
>> Date: Thu, 26 May 2016 00:13:28 +0200
>> 
>> Any news about this issue? I am still using an Emacs build with 
>> Stephen's patch and I see no problem...
>
> Would someone please install that?

Done in commit 16be3e90545972dec16014253a843229d5bdf388 to master and
closing the bug.

Steve Berman