Visual artifact 1px vertical white line right side of the wallpaper with Ambiance/Radiance

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Thanks. Please try removing your extensions in case they are the cause:

  cd ~/.local/share/gnome-shell/
  rm -rf extensions

and then log in again.

Done. There is no change. The artifact is still present.

In the past I had seen a 1px gap down the right side of the wallpaper on Intel, but I think that was only during desktop zoom and I can't reproduce it today. Whatever the cause was might still be present and might only be manifesting with AMD graphics. Just a theory.

That could very well be the case. It does not appear when the desktop is initially loaded, but shortly after a refresh(?). If I keep the second monitor (TV) active, the scaling workaround does not work - the lines on both monitors come back when set back to 100% scale. If I click the desktop the line disappears from the monitor that was clicked - and comes back once the desktop is no longer focused. An annoying little thing that I'm sure is equally annoying to figure out.

Can you attach a photo or video of the issue?

Picture one, default when the desktop is no focused.

Picture two, after the desktop was scaled to 125% and then back to 100%, and the Settings window is not closed, yet (the white line disappears after the Settings window is closed).

Single monitor examples. The same happens when I activate the second monitor, with the exception that the white lines are both on the second display before closing the Settings (if I recall correctly). Would you like pictures of the two monitor setup, as well? Or do these two pictures demonstrate the artifact well enough so that they are not needed?

Thanks, that's enough info.

My immediate thought is that it might relate to mutter's background rendering code which aggressively tries to limit the number of wallpaper pixels it redraws each frame. Maybe there's an off-by-one mistake in that code.

I would not know. Nevertheless, I consider this bug report as a heads up in case you encounter it or something similar in the future. It is nothing critical, and the workaround does work, even though applying it every time I log in to Ubuntu will probably get tedious in the long run. Does this even affect others? No idea.

I've not seen any other reports of this bug.

If it is mostly only visible on AMD graphics and less than 6% of Ubuntu users are using AMD graphics, intersected with the small proportion of users who are willing to report bugs instead of ignoring them, it's a small intersection so unsurprising it took this long to be reported.

I need to switch to AMD to work on another bug soon so I will check on this one too.

An update since I have a new computer with a fresh, clean install of Ubuntu 22.04.3 LTS...this may very likely be a theme issue. It is not present when using the default theme. It appears when trying to change the Applications theme to - for example - Ambiance (the one I would prefer).

Consider this solved?

If the bug still occurs with any theme then we should keep it open.

I can reproduce the problem on two computers, both running fully-updated 24.04 beta.

For me, the bug appears if all of these circumstances are met:
- /org/gnome/desktop/interface/gtk-theme is Ambiance or Radiance
- one of the windows is focused
- the Desktop Icons NG (DING) extension is enabled in extension-manager

The first two matches earlier observations here, the third one looks like new information.

As you toggle the DING extension, this vertical 1px wide bar quickly rushes to/from the right edge of the screen. It suggests that it's not that the background isn't drawn properly, it's rather that DING explicitly draws this line on top of it (and shows a brief animation when getting enabled/disabled).

This also aligns with the observation that a window needs to be focused -- probably if focus is taken away from windows by clicking on the background then DING receives the focus, and apparently it's DING's unfocused state combined with Ambi/Radi themes that results in this glitch.

I'm almost certain that this is a bug either in DING or in the Ambi/Radi themes.

It's probably not related to the graphics card. One of these computers has an NVidia (nouveau driver), the other one has ATI Radeon. Scaling is probably unrelated, too. The glitch is seen on both Xorg and Wayland.

Looks remotely related: https://askubuntu.com/questions/927893/white-line-on-left-of-ubuntu-launcher-when-desktop-icons-enabled

I've modified /usr/share/themes/Ambiance (recursively), search-replacing all the "1px" to "10px", and (after logging out and back in) the line at the desktop's right edge looks about 10px wide.

So we're on the right track.

It's /usr/share/themes/{Amb,Rad}iance/gtk-3.20/gtk-widgets.css line 18:

  .background:backdrop {
      color: @backdrop_fg_color;
      box-shadow: inset -1px 0 shade (@bg_color, 0.94);
                        ^^^^
  }

Change that to 0 (or I guess you could remove the entire line or even the entire segement).

(I'm not a gtk css expert by any means, I don't know if this is _the_ proper way to solve this issue, nor whether it causes problems elsewhere.)

Wow, I didn't realize that people were still trying to use Ambiance and Radiance (from source package ubuntu-themes) with Ubuntu 24.04 LTS.

Would you like to submit a patch and subscribe ubuntu-sponsors?

Without root access (or being worried of an update overwriting the changes), this is how I could fix it as a user.

I've placed this in my ~/.config/gtk-3.0/gtk.css file:

  window.desktopwindow.background:backdrop {
    box-shadow: none;
  }

Don't ask why it's gtk-3.0 and not 3.20 or 4.0, I have no clue. [Update: I've figured out, gnome-shell-extension-desktop-icons-ng is a gtk3 app.]

The "window.desktopwindow" selector is something I've found in gnome-shell-extension-desktop-icons-ng's source. It's a more specific selector than the one in the theme, hence takes precedence over that. Without this selector they would be of the same precedence, and for some reason the one in the theme is used, the one in the user's config is ignored. And GTK doesn't seem to support "!important".

This extra selector luckily gives some confidence that the rule probably doesn't have an effect on others. (However, the disclaimer from my previous comment still applies.)

In extension-manager toggle the said extension back and forth to apply the change.

> Wow, I didn't realize that people were still trying to use Ambiance and Radiance (from source package ubuntu-themes) with Ubuntu 24.04 LTS.

I'd love to switch to Adwaita or Yaru, but I prefer to have dark window header and light window contents, and I haven't found yet how to do it in those.

(I'm also in the process of evaluating whether I want to leave Unity behind and switch to GNOME or its Ubuntu desktop flavor... that's how I came across this issue.)

> Would you like to submit a patch and subscribe ubuntu-sponsors?

The thing is, I'm not confident that I'm doing the right thing and not causing side effects elsewhere. I'm even unsure at this moment whether the fix needs to go into the theme, or the extension should ship the override.

I'd much rather prefer if someone familiar with GTK theming/CSS could take over from here.

Ambiance/Radiance are basically unmaintained so if there's a bug it would probably be easier for you to propose a fix then trying to find someone else to do it.

If you have concerns about the extension, I suggest filing an upstream issue:
https://gitlab.com/rastersoft/desktop-icons-ng/-/issues

Let's see what DING devs think: https://gitlab.com/rastersoft/desktop-icons-ng/-/issues/315

DING developer's response is basically that they clear the "decorated" property of the window, and the theme should handle that.

Based on this, I think the correct solution for the Ambiance/Radiance themes is to further restrict the offending block with the ".decorated" selector, i.e.

  .background.decorated:backdrop {
      color: @backdrop_fg_color;
      box-shadow: inset -1px 0 shade (@bg_color, 0.94);
  }

or for the user's own gtk.css to select for its negated ":not(.decorated)" for the workaround:

  .background:not(.decorated):backdrop {
      box-shadow: none;
  }

> .background.decorated:backdrop {

This still doesn't feel entirely good. The background can be applied to a window, in which case it needs to be restricted to the .decorated property; but I presume it can also be applied to many other GTK widgets in which case it shouldn't be restricted, at least in order not to change the current behavior because I don't know how to test any change.

So if I'd like to perform the smallest possible change, the smallest possible impact of the change (preferably: no other change than fixing this bug) then I'd need to select for "either window.decorated or something non-window", which I'm not sure how to express in CSS (at least not in a single step).

Maybe leave the first block untouched, and then revert its effect in the next rule for undecorated windows (effectively moving the above-proposed user workaround into the main theme, restricted to the element being a window).

So the theme itself would go like (the first block is unchanged, the second one is newly added):

  .background:backdrop {
      color: @backdrop_fg_color;
      box-shadow: inset -1px 0 shade (@bg_color, 0.94);
  }

  window.background:not(.decorated):backdrop {
      box-shadow: none;
  }

Dunno how much it makes sense. Would really love to get it reviewed by a CSS expert :)