bug#21750: show-paren-mode erroneously highlights the left margin

bug#21750: show-paren-mode erroneously highlights the left margin

[Hinrik Örn Sigurðsson]

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Hi there. I discovered that under certain conditions, show-paren-mode
will highlight the entire left margin of the window when one of the
parentheses is located at the beginning of a line. If you do "M-x
eval-buffer" on the following code, it will reproduce the issue in
emacs -Q (in both the terminal and GUI):

    (linum-mode)
    (custom-set-faces
      '(linum ((t (:foreground "black" :weight bold)))))
    (show-paren-mode)

It happens when linum-mode (or nlinum-mode) is active, but only if the
'linum face has been customized. Attached is a screenshot
demonstrating the overzealous highlighting.


In GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.9)
 of 2015-03-21 on kissel, modified by Debian
System Description:    Ubuntu 15.04

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Loading /etc/emacs/site-start.d/50autoconf.el (source)...done
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bug#21750: show-paren-mode erroneously highlights the left margin

[Eli Zaretskii]

> Date: Sat, 24 Oct 2015 19:28:24 +0200
> From: Hinrik Örn Sigurðsson
> 	<hinrik.sig@gmail.com>
> 
> Hi there. I discovered that under certain conditions, show-paren-mode
> will highlight the entire left margin of the window when one of the
> parentheses is located at the beginning of a line. If you do "M-x
> eval-buffer" on the following code, it will reproduce the issue in
> emacs -Q (in both the terminal and GUI):
> 
>     (linum-mode)
>     (custom-set-faces
>       '(linum ((t (:foreground "black" :weight bold)))))
>     (show-paren-mode)
> 
> It happens when linum-mode (or nlinum-mode) is active, but only if the
> 'linum face has been customized. Attached is a screenshot
> demonstrating the overzealous highlighting.

I think this is a side effect of the general feature whereby an
overlay string "inherits" the face of the surrounding buffer text.
It doesn't matter whether the overlay string is displayed in the
window's display margin or in the text area.

I can understand how this could be unexpected in this particular case,
and might look like a bug, but it's actually the display engine
functioning as designed.  Resetting the face of the overlay string
back to the default might disrupt lots of existing code.

bug#21750: show-paren-mode erroneously highlights the left margin

[Eli Zaretskii]

> Date: Sat, 24 Oct 2015 23:07:03 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 21750@debbugs.gnu.org
> 
> I think this is a side effect of the general feature whereby an
> overlay string "inherits" the face of the surrounding buffer text.
> It doesn't matter whether the overlay string is displayed in the
> window's display margin or in the text area.

You can work around this in this case if you modify your face
customization as follows:

    (custom-set-faces
      '(linum ((t (:foreground "black" :background "white" :weight bold)))))

IOW, don't let the 'linum' face inherit the background color from the
surrounding text.

bug#21750: show-paren-mode erroneously highlights the left margin

[Stephen Berman]

On Sat, 24 Oct 2015 23:22:36 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> Date: Sat, 24 Oct 2015 23:07:03 +0300
>> From: Eli Zaretskii <eliz@gnu.org>
>> Cc: 21750@debbugs.gnu.org
>> 
>> I think this is a side effect of the general feature whereby an
>> overlay string "inherits" the face of the surrounding buffer text.
>> It doesn't matter whether the overlay string is displayed in the
>> window's display margin or in the text area.

Is this clearly documented in the Emacs Lisp manual?  If not, I think it
should be.  (The only places I found that mention overlays and faces
together are 37.9.2 Overlay Properties and 37.12.4 Displaying Faces, but
in neither is this effect of face inheritance clearly stated, AFAICS.)

> You can work around this in this case if you modify your face
> customization as follows:
>
>     (custom-set-faces
>       '(linum ((t (:foreground "black" :background "white" :weight bold)))))
>
> IOW, don't let the 'linum' face inherit the background color from the
> surrounding text.

It would also be very helpful to document this workaround.

Steve Berman

bug#21750: show-paren-mode erroneously highlights the left margin

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: hinrik.sig@gmail.com,  21750@debbugs.gnu.org
> Date: Sat, 24 Oct 2015 23:54:02 +0200
> 
> >> I think this is a side effect of the general feature whereby an
> >> overlay string "inherits" the face of the surrounding buffer text.
> >> It doesn't matter whether the overlay string is displayed in the
> >> window's display margin or in the text area.
> 
> Is this clearly documented in the Emacs Lisp manual?  If not, I think it
> should be.

Which part(s) of what I said you think should be clearly documented?
The part about display/overlay strings using the face of the
"underlying" text, or the fact that this applies to strings displayed
on the display margins?

> (The only places I found that mention overlays and faces
> together are 37.9.2 Overlay Properties and 37.12.4 Displaying Faces, but
> in neither is this effect of face inheritance clearly stated, AFAICS.)

"Displaying Faces" is the place that should make it clear.  It
describes the order in which Emacs determines a face appropriate for
displaying a given character, using the various possible sources.  If
something there is unclear or missing, please elaborate.  (It is clear
to me, but I'm not the typical reader of those parts, obviously ;-)

> > You can work around this in this case if you modify your face
> > customization as follows:
> >
> >     (custom-set-faces
> >       '(linum ((t (:foreground "black" :background "white" :weight bold)))))
> >
> > IOW, don't let the 'linum' face inherit the background color from the
> > surrounding text.
> 
> It would also be very helpful to document this workaround.

Once again, the above section says that specifying the face takes the
specified attributes from that face, and merges them with faces from
lower-priority sources in order to determine unspecified attributes.
In this case, the background color is not specified by the customized
'linum' face, so Emacs takes it from the buffer text.  I'm open to
suggestions for how to make this more evident.

In any case, I take it that you agree that what Emacs does is the only
reasonable thing in this case, right?

bug#21750: show-paren-mode erroneously highlights the left margin

[Stephen Berman]

On Sun, 25 Oct 2015 20:37:28 +0200 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: hinrik.sig@gmail.com,  21750@debbugs.gnu.org
>> Date: Sat, 24 Oct 2015 23:54:02 +0200
>> 
>> >> I think this is a side effect of the general feature whereby an
>> >> overlay string "inherits" the face of the surrounding buffer text.
>> >> It doesn't matter whether the overlay string is displayed in the
>> >> window's display margin or in the text area.
>> 
>> Is this clearly documented in the Emacs Lisp manual?  If not, I think it
>> should be.
>
> Which part(s) of what I said you think should be clearly documented?
> The part about display/overlay strings using the face of the
> "underlying" text, or the fact that this applies to strings displayed
> on the display margins?

I guess the former, but I want to check the latter again, because I
have a recollection of seeing a difference, but I won't have time to try
tracking it down for several days.  I'll get back to you on this and the
rest of your questions.  Thanks.

Steve Berman

bug#21750: show-paren-mode erroneously highlights the left margin

[Eli Zaretskii]

I think this bug can be closed now.

bug#21750: show-paren-mode erroneously highlights the left margin

[Hinrik Örn Sigurðsson]

On Sat, Oct 24, 2015 at 10:22 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> You can work around this in this case if you modify your face
> customization as follows:
>
>     (custom-set-faces
>       '(linum ((t (:foreground "black" :background "white" :weight bold)))))
>
> IOW, don't let the 'linum' face inherit the background color from the
> surrounding text.

Unfortunately that doesn't work in my case. The desired background
color is pure black (#000000). However both "black" and "#000000"
result in Emacs choosing the 1st color of the terminal's 16-color
palette, even when running in 256-color mode. Though pure black is my
terminal's default background color (which is independent of the
palette), it is not present in the 16-color palette.

So there's no way of explicitly specifying this background color in
Emacs unless I change my terminal's color palette, which would affect
the appearance of other terminal programs.

bug#21750: show-paren-mode erroneously highlights the left margin

[Eli Zaretskii]

> Date: Sun, 25 Oct 2015 01:53:34 +0200
> From: Hinrik Örn Sigurðsson <hinrik.sig@gmail.com>
> Cc: 21750@debbugs.gnu.org
> 
> > You can work around this in this case if you modify your face
> > customization as follows:
> >
> >     (custom-set-faces
> >       '(linum ((t (:foreground "black" :background "white" :weight bold)))))
> >
> > IOW, don't let the 'linum' face inherit the background color from the
> > surrounding text.
> 
> Unfortunately that doesn't work in my case.

By "don't work" what exactly do you mean?  Do you still see the
greenish background of the show-paren face on the line numbers?  Or do
you see something else?

> The desired background color is pure black (#000000). However both
> "black" and "#000000" result in Emacs choosing the 1st color of the
> terminal's 16-color palette, even when running in 256-color mode.

That's a different issue, but I think it, too, can be solved, see
below.

(Btw, I wonder why you use a black foreground for this face if your
background is also black.  What am I missing here?)

> Though pure black is my terminal's default background color (which
> is independent of the palette), it is not present in the 16-color
> palette.

That's strange: are you saying that a 16-color xterm doesn't have
"black" among the colors it supports?  I don't think I've ever seen
that; perhaps the settings for xterm on your system deliberately
define "black" as something whose RGB value is not #000000?

> So there's no way of explicitly specifying this background color in
> Emacs unless I change my terminal's color palette, which would affect
> the appearance of other terminal programs.

What happens if you specify the 'linum' face to inherit the background
from your 'default' face, like this:

  (custom-set-faces
   '(linum ((t (:inherit default :foreground "black" :weight bold)))))

Does this produce good results?

bug#21750: show-paren-mode erroneously highlights the left margin

[Hinrik Örn Sigurðsson]

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On Sun, Oct 25, 2015 at 7:39 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> (Btw, I wonder why you use a black foreground for this face if your
> background is also black.  What am I missing here?)


> That's strange: are you saying that a 16-color xterm doesn't have
> "black" among the colors it supports?  I don't think I've ever seen
> that; perhaps the settings for xterm on your system deliberately
> define "black" as something whose RGB value is not #000000?

"black" to emacs is really color 0, which I have defined to be a dark
grey color (#2e3436) in my terminal (see attached screenshot). I do that
because it is a useful color to have rather than duplicating the #000000
color which can already be achieved by specifying no background color.

For the record, I'm using gnome-terminal with the Tango color palette.
When that palette was first intoduced, it had color 0 set to #2e3436,
but at some point they changed it to #000000 (which arguably makes it
non-Tango as #000000 is not found in
http://tango.freedesktop.org/Tango_Icon_Theme_Guidelines). But I still
customize color 0 to #2e3436, to have a wider range of colors.

>
>> So there's no way of explicitly specifying this background color in
>> Emacs unless I change my terminal's color palette, which would affect
>> the appearance of other terminal programs.
>
> What happens if you specify the 'linum' face to inherit the background
> from your 'default' face, like this:
>
>   (custom-set-faces
>    '(linum ((t (:inherit default :foreground "black" :weight bold)))))
>
> Does this produce good results?

This actually works. The background color of the parenthesis no longer
bleeds into the margin. Thanks!

[-- Attachment #2: colors.png --]
[-- Type: image/png, Size: 42406 bytes --]

bug#21750: show-paren-mode erroneously highlights the left margin

[Johan Bockgård]

Hinrik Örn Sigurðsson <hinrik.sig@gmail.com> writes:

> Unfortunately that doesn't work in my case. The desired background
> color is pure black (#000000). However both "black" and "#000000"
> result in Emacs choosing the 1st color of the terminal's 16-color
> palette, even when running in 256-color mode. Though pure black is my
> terminal's default background color (which is independent of the
> palette), it is not present in the 16-color palette.
>
> So there's no way of explicitly specifying this background color in
> Emacs unless I change my terminal's color palette, which would affect
> the appearance of other terminal programs.

The terminal's foreground and background colors can be referred to as
"unspecified-fg" and "unspecified-bg". (Cf. M-x describe-face RET
default RET)