Option to follow system theme (night/day, dark/light) gone missing on Linux

[victorz]

Steps to reproduce

1. Open up Telegram Desktop on Linux (X11)
2. Go to chat settings
3. The option to follow the system theme used to be there, but is now missing

I am installing from the Arch package repositories, so the source code is just downloaded and built and packaged for Arch, according to this PKGBUILD:

https://gitlab.archlinux.org/archlinux/packaging/packages/telegram-desktop/-/blob/main/PKGBUILD

Expected behaviour

The theme should be able to follow the system theme, dark/light mode, but it cannot anymore.

Actual behaviour

There's no setting to enable following system theme anymore, and the client has stopped auto-switching from day to night theme and vice versa when enabling dark/light theme.

Operating system

Arch Linux

Version of Telegram Desktop

4.9.5

Installation source

Other (unofficial) source

Crash ID

No response

Logs

No response

[Aokromes]

plz try downloading official static (non-flakpak, non-snap) version from https://desktop.telegram.org

[ilya-fedin]

This means Qt on your system doesn't support it (QStyleHints::colorScheme should return not Unknown) :(
Try official binary with patched Qt

[victorz]

Thank you for your replies!

I tried the official (non-flatpak, non-snap) version from https://desktop.telegram.org/, and that works fine. So something clearly changed in the building of the Arch Linux package or a regression was introduced in Qt (is that the implication?).

The official static binary works regardless of the presence of xdg-desktop-portal-kde systemd unit.

Do you know if there's a way that I can check whether this is a problem with Qt on my system or with the way they build the package? How can I be sure which piece of software is in the wrong here?

[ilya-fedin]

That's not a change in the package, it's a change in tdesktop. There was a portal listener but it was removed due to #26370. It's better to not to have this feature consistently than having a bugged behavior with difference between on start and in runtime.

You need to ask your Qt Platform Theme provider (it could be either Qt itself or a third-party Qt plugin like qt6ct) to support QPlatformTheme::colorScheme() method and call QWindowSystemInterface::handleThemeChange() once the color scheme changes.