Not working Qt6 Qt::ColorScheme

[redtide]

Hello,
I'm trying to update a widgets set project with the possibility to react on theme changes.

I see that from 6.5, Qt has a new colorScheme property, to know which color scheme is currently in use; Dark, Light or Unknown (here an interesting article in Windows 11 context).

Unfortunately in LXQt it seems to return always Unknown, so I wanted to ask you, as Kvantum and LXQt developer, if there is something that one of them can do about it, or if it's a Qt (XCB at least) non working feature for some reason and/or context.

For now it seems I get successful results by using a function that checks the relative luminance of QPalette::Window' color, together with the (undocumented) QEvent::ThemeChange event type, which works also with Qt5.

[tsujan]

It's because the Qt6 branch of lxqt-qtplugin (provided that you have it) doesn't support colorScheme() yet and Qt → qplatformtheme.cpp contains

Qt::ColorScheme QPlatformTheme::colorScheme() const
{
    return Qt::ColorScheme::Unknown;
}

That being said, even when we add it to the Qt6 branch of lxqt-qtplugin, it won't be applicable with style plugins like Kvantum, which have their own palette and don't follow the platform palette.

[tsujan]

... so I would expect Qt to return a Dark color scheme

As far as I've checked the code of Qt 6.6, I think that shouldn't happen, because Qt consults the platform, not the actual palette. And yes, it's an inconsistency (but not the only one I've encountered in Qt).

My guess is that many Qt devs don't know of possibilities beyond Fusion-like themes.

[redtide]

I have no much an idea how it works in Qt, ideally to me Kvantum should tell to LXQt plugin which color mode is using (from theme or custom palette) and then the plugin report it to Qt, but IDK how this should be done.

From code I see in src/plugins/platformthemes there are a gtk3 and a xdgdesktopportal plugins, where IIUC gtk3 search for a "dark" value substring in the gtk-theme-name key on $HOME/.config/gtk3.0/settings.ini file, and the color-scheme key in org.freedesktop.portal.Settings.

Something similar to GTK3 is made also for KDE.

[tsujan]

Being a style plugin, Kvantum reports what QCommonStyle does. It's up to Qt devs to add a method to QStyle for getting a "Qt::ColorScheme".

Breeze and Fusion are irrelevant as far as Kvantum is concerned, let alone GTK3.

[redtide]

Thank you for your time and clarifications. Should I close this?

[tsujan]

You're very welcome.

As this contains information that may be useful to others (well, it was useful to me), I'd prefer to keep it open. Generally, discussions don't need to be closed.