bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Yanchi Tóth]

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White background color flashes on startup, before background color from
config takes effect in parts of the rectangle occupied by Emacs. Didn't
happen on 28.2, but does happen on both this RC, and the other 29
pretests. See video attachment, and please let me know if I can be of any
help.

In GNU Emacs 29.1 (build 2, x86_64-w64-mingw32) of 2023-07-23 built on
 AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19045
System Description: Microsoft Windows 10 Pro (v10.0.2009.19045.3208)

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bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Eli Zaretskii]

> From: Yanchi Tóth <yanchi.toth@gmail.com>
> Date: Tue, 25 Jul 2023 10:56:31 +0200
> 
> White background color flashes on startup, before background color from
> config takes effect in parts of the rectangle occupied by Emacs. Didn't
> happen on 28.2, but does happen on both this RC, and the other 29
> pretests. See video attachment, and please let me know if I can be of any help.

Please show a minimal init file that can be used to reproduce this
behavior.

Thanks.

bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Yanchi Tóth]

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Hi!

I managed to reproduce with this fairly minimal configuration:

(set-background-color "#191919")
(set-foreground-color "burlywood")

Because the config is much smaller, the flash of white is much shorter, but
still noticeable.


On Tue, Jul 25, 2023 at 2:43 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Yanchi Tóth <yanchi.toth@gmail.com>
> > Date: Tue, 25 Jul 2023 10:56:31 +0200
> >
> > White background color flashes on startup, before background color from
> > config takes effect in parts of the rectangle occupied by Emacs. Didn't
> > happen on 28.2, but does happen on both this RC, and the other 29
> > pretests. See video attachment, and please let me know if I can be of
> any help.
>
> Please show a minimal init file that can be used to reproduce this
> behavior.
>
> Thanks.
>

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bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Eli Zaretskii]

> From: Yanchi Tóth <yanchi.toth@gmail.com>
> Date: Tue, 25 Jul 2023 15:32:33 +0200
> Cc: 64846@debbugs.gnu.org
> 
> I managed to reproduce with this fairly minimal configuration:
> 
> (set-background-color "#191919")
> (set-foreground-color "burlywood")
> 
> Because the config is much smaller, the flash of white is much shorter, but still noticeable.

Instead of the above, try adding to your early-init file the
following:

  (add-to-list 'default-frame-alist
	       '(background-color . "#191919")
	       '(foreground-color . "burlywood"))

bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Yanchi Tóth]

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Hmmm. I tried adding the early init file with what you suggest, but the
situation stays the same

Note that if I make the frame larger at startup, the bright rectangle stays
pretty much the same size,
which hints at this being related to repainting dirty areas? Maybe when
Windows sends the first WM_ERASEBKGND,
the frame background color is not set yet, so Emacs repaints with white?

I don't think this is a release blocker, and I could investigate this later
myself, if it would help.

On Tue, Jul 25, 2023 at 4:32 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Yanchi Tóth <yanchi.toth@gmail.com>
> > Date: Tue, 25 Jul 2023 15:32:33 +0200
> > Cc: 64846@debbugs.gnu.org
> >
> > I managed to reproduce with this fairly minimal configuration:
> >
> > (set-background-color "#191919")
> > (set-foreground-color "burlywood")
> >
> > Because the config is much smaller, the flash of white is much shorter,
> but still noticeable.
>
> Instead of the above, try adding to your early-init file the
> following:
>
>   (add-to-list 'default-frame-alist
>                '(background-color . "#191919")
>                '(foreground-color . "burlywood"))
>

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bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Eli Zaretskii]

> From: Yanchi Tóth <yanchi.toth@gmail.com>
> Date: Wed, 26 Jul 2023 10:32:54 +0200
> Cc: 64846@debbugs.gnu.org
> 
> Hmmm. I tried adding the early init file with what you suggest, but the situation stays the same

The other possibility is to set the background and foreground colors
in the X resources.  On Windows, you can put these "X resources" in
the Registry, and Emacs will follow them.  See the node "X Resources"
in the Emacs user manual for the details.

> Note that if I make the frame larger at startup, the bright rectangle stays pretty much the same size,
> which hints at this being related to repainting dirty areas? Maybe when Windows sends the first
> WM_ERASEBKGND,
> the frame background color is not set yet, so Emacs repaints with white?

Maybe.  I don't know enough about the low-level details of the Emacs
display on Windows to tell.

> I don't think this is a release blocker, and I could investigate this later myself, if it would help.

It definitely won't block the release of Emacs 29.1.

bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Maybe.  I don't know enough about the low-level details of the Emacs
> display on Windows to tell.

Could this be related to the new MS-Windows double buffering code?

When WM_ERASEBKGND arrives and double buffering is enabled, Emacs simply
copies the back buffer contents to the front buffer HWND.  Perhaps this
bug is a result of the copy transpiring in between the creation of the
back buffer, and when redisplay first clears the frame with its
background color.  In that case, the solution is for the back buffer to
always be filled with the frame background color after every time it is
created, instead of whichever color CreateCompatibleBitmap opts to fill
it with by default.

Yachani, what happens if you place:

  (setq w32-disable-double-buffering t)

in your early-init.el?

bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Yanchi Tóth]

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Po,

> what happens if you place:
>
>  (setq w32-disable-double-buffering t)
>
> in your early-init.el?

It helps, but doesn't remove the bright flash altogether. Hard to tell, but
I believe it shortens the time of the flash to just one frame. For
comparison: 28.2 doesn't have any perceptible bright flash.

I will have some time over the weekend to look at the code, but I haven't
seen the Emacs codebase yet, so I might be taking more than I can handle.
Can you give me any pointers where to look?

On Wed, Jul 26, 2023 at 2:35 PM Po Lu <luangruo@yahoo.com> wrote:

> Eli Zaretskii <eliz@gnu.org> writes:
>
> > Maybe.  I don't know enough about the low-level details of the Emacs
> > display on Windows to tell.
>
> Could this be related to the new MS-Windows double buffering code?
>
> When WM_ERASEBKGND arrives and double buffering is enabled, Emacs simply
> copies the back buffer contents to the front buffer HWND.  Perhaps this
> bug is a result of the copy transpiring in between the creation of the
> back buffer, and when redisplay first clears the frame with its
> background color.  In that case, the solution is for the back buffer to
> always be filled with the frame background color after every time it is
> created, instead of whichever color CreateCompatibleBitmap opts to fill
> it with by default.
>
> Yachani, what happens if you place:
>
>   (setq w32-disable-double-buffering t)
>
> in your early-init.el?
>

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bug#64846: Emacs 29 RC on Windows: White background color flashes on startup, before background color from config takes effect on Windows

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Yanchi Tóth <yanchi.toth@gmail.com> writes:

> It helps, but doesn't remove the bright flash altogether. Hard to
> tell, but I believe it shortens the time of the flash to just one
> frame.  For comparison: 28.2 doesn't have any perceptible bright
> flash.
>
> I will have some time over the weekend to look at the code, but I
> haven't seen the Emacs codebase yet, so I might be taking more than I
> can handle. Can you give me any pointers where to look?

See w32fns.c (specifically, `w32_wnd_proc', which responds to
WM_ERASEBKGND messages), `get_frame_dc' in w32xfns.c (which creates the
back buffer), and w32term.c, where you will find a large table of
function pointers that implement the interfaces redisplay uses to draw
glyphs on MS Windows under the name `w32_redisplay_interface'.

I'm hardly an expert in MS Windows graphics, BTW.  You likely know more
than I do.

Thanks.