bug#39265: 26.3; format-mode-line with FACE argument

bug#39265: 26.3; format-mode-line with FACE argument

[ynyaaa]

#'format-mode-line assumes all characters of a string have a same 'face
property value and checks only the first character.

  (format-mode-line (concat "A" (propertize "XYZ" 'face 'region)) 'highlight)
  => #("AXYZ" 0 1 (face highlight) 1 4 (face highlight))

#'format-mode-line merges the 'face property value of the first
character of a string with FACE argument. It is not ducumented at all.

  (format-mode-line (propertize "ABC" 'face 'region) 'highlight)
  => #("ABC" 0 3 (face (region highlight)))

#'format-mode-line called with FACE=t should use 'mode-line or
'mode-line-inactive instead of 't, but it treats 't as a face.

  (format-mode-line "ABC" t)
  => #("ABC" 0 3 (face t))


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 10.0.18363
Recent messages:
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Undo!

Configured using:
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 --without-compress-install 'CFLAGS=-O2 -static -g3''

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TOOLKIT_SCROLL_BARS THREADS LCMS2

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bug#39265: 26.3; format-mode-line with FACE argument

[Eli Zaretskii]

tags 39265 notabug
thanks

> From: ynyaaa@gmail.com
> Date: Sat, 25 Jan 2020 00:05:25 +0900
> 
> 
> #'format-mode-line assumes all characters of a string have a same 'face
> property value and checks only the first character.

Yes.  The reason is performance: the mode line is redrawn very
frequently, basically, after every command.

This is not a bug, this is the intended behavior.

bug#39265: 26.3; format-mode-line with FACE argument

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

> tags 39265 notabug
> thanks
>
>> From: ynyaaa@gmail.com
>> Date: Sat, 25 Jan 2020 00:05:25 +0900
>> 
>> 
>> #'format-mode-line assumes all characters of a string have a same 'face
>> property value and checks only the first character.
>
> Yes.  The reason is performance: the mode line is redrawn very
> frequently, basically, after every command.
>
> This is not a bug, this is the intended behavior.

I'm consequently also closing this bug report.

Best regards,
Stefan Kangas