Add KDE theming support

[ilya-fedin]

KDE applications are a notable part of the popular Qt software in the Linux world.
Sadly, they have their own theming add-ons that are applied through QPlatformTheme.

This adds support for all aspects of the KDE applications theming present in the plasma-integration QPlatformTheme:

• KDE's QtQuick style bridging the currently set QtWidgets style (you may want to get QtQuick style choice in the settings UI in the future instead)
• KDE's color schemes (the most important part as KDE applications don't fallback to QtQuick style/QtWidgets style/QPalette for colors)
• KDE's icon engine applying KDE's color schemes to icons (to not to have dark icons on dark background)
• Writing widget style and icon theme to kdeglobals for applications using KStyleManager

I also have a qt5ct variant of this patch but I don't have a SourceForge account nor SVN experience to send it there... If you wish I can upload the patch here as a file.

Fixes #38

[spikespaz]

Do you know the name of the Qt module that provides QQuickStyle? I thought it was qtdeclarative but maybe not. It's failing to find that include. Trying to build with Nix.

[ilya-fedin]

@spikespaz I have a package (and a module) for nix in https://github.com/ilya-fedin/nur-repository. Regarding the Qt module, yeah, it's qtdeclarative.

[mechakotik]

Tried it out, most KDE apps work fine for me, but Plasma system monitor doesn't follow the color scheme correctly and looks like this:

Here are the logs:

kf.config.core: Watching absolute paths is not supported "/home/mecha/.local/share/color-schemes/BreezeDarkTeal.colors"
kf.kirigami.platform: Failed to find a Kirigami platform plugin for style "Fusion"
qrc:/qt/qml/org/kde/ksysguard/page/EditablePage.qml?page=history.page:223:9: QML Loader: Binding loop detected for property "height"
file:///usr/lib/qt6/qml/org/kde/kirigami/Dialog.qml:334:18: QML ScrollView: Binding loop detected for property "calculatedImplicitWidth"
file:///usr/lib/qt6/qml/org/kde/kirigami/Dialog.qml:386:33: QML Binding: Binding loop detected for property "target"
qml: Implementation matching https://systemd.io/DESKTOP_ENVIRONMENTS/ was not found. ApplicationsView will not be available

KDE system settings app has the same problem.

[ilya-fedin]

Should be ok now

[Hasshu]

@trialuser02 Is there anything blocking this PR?

[spion]

FWIW I tried this patch and it seems to affect the "dark mode" setting on my system in browsers like Chromium (that is, dark KDE themes don't seem to make chromium think the system is set into "dark mode", but color schemes do)

Its also probably a good idea to disable copying the KDE themes as the created copies don't work

[ilya-fedin]

Its also probably a good idea to disable copying the KDE themes as the created copies don't work

What copying are you talking about?

[spion]

What copying are you talking about?

Next to the color scheme dropdown, there is an actions dropdown menu with a "Create a copy" item. I wanted to use it to try and tweak the KDE scheme and see if I can get it to trigger "dark mode". This created a copy of the file under ~/.config/qt6ct/colors that doesn't work when selected.

[ilya-fedin]

Next to the color scheme dropdown, there is an actions dropdown menu with a "Create a copy" item.

Should be hidden now

[ilya-fedin]

I wonder whether we want to handle that new thing of KDE frameworks reading kdeglobals directly instead of following Qt settings (icon theme/widget style)?

That would mean modifying kdeglobals thus changing the settings of Plasma session (if one has it)...

[ilya-fedin]

I wonder whether we want to handle that new thing of KDE frameworks reading kdeglobals directly instead of following Qt settings (icon theme/widget style)?

Implemented it. Also made PRs with some changes that should make qt6ct behave way less buggy:
#52
#53
#54
#55

But it seems there are low changes of them being merged given maintainer's inactivity...

[Prepodobnuy]

@ilya-fedin pls update the aur package <3

[ilya-fedin]

It has the latest version of the patch...

[ilya-fedin]

@Prepodobnuy you may need to rebuild the package with the new Qt version

[Hasshu]

@Conan-Kudo @grulja Do you think we could have this in Fedora?

[ilya-fedin]

I've splitted theming bits to separate commits. They are also now optional so qmake build works (but builds without any KDE theming bits of course).

idk whether anyone needs it but I splitted the commits refactoring Qt6CT::{resolvePath,loadColorScheme} to a separate PR (#58) and while working on optionalisation, I also made cmake build to support building without QtWidgets (#57).

[ilya-fedin]

If anyone needs all commits from all PRs rebased on top of each other, based on 0.9, as in my packages, here they are: https://github.com/ilya-fedin/qt6ct/tree/shenanigans

[ilya-fedin]

With KF 6.8, applications using KColorSchemeManager should start to follow palette changes in runtime just like applications not using it

[ilya-fedin]

FWIW, I also tried (unsuccessfully) to communicate the kvantum dev that he does things that make the palette stuck even after switching out of Kvantum to other style in runtime, both with vanilla's qt6ct AA_SetPalette check and my QWindowSystemInterface::handleThemeChange approach (as it basically checks AA_SetPalette under the hood, too). The discussion is at tsujan/Kvantum#1018

[Gigas002]

Sorry, I don't know if it's related to KDE support in qt6ct, but dolphin misses theming of icons, when you target the search path, e.g. here the above icon is not themed, while all the others are themed correctly.

This works fine when running on KDE, only affects other compositors.

[ilya-fedin]

Perhaps your icon theme just has no such icon so it falls back to Breeze?

[Gigas002]

Hmm, I think you may be right. I've tested some other themes and all of these are themed correctly.
I thought this behavior is weird, since this works correct on KDE session, but I guess I should take a look at the theme itself. Sorry for the noise.

[ilya-fedin]

Maybe it just looks similar but still from some other icon theme on KDE session?

[Gigas002]

I found the missing icon, it was the folder-open.svg. Placed it under /usr/share/icons/theme_name/Places/folder-open.svg and it resolved the issue.