A lot has been happening with Union recently, so it is time for another update. As a reminder, Union is a project to build a new style engine for applications that tries to unify all the different ways Qt and KDE applications can be styled.
Union in Plasma 6.7
As mentioned in the [spring update], Union had its first release as part of the Plasma 6.7 release. Before release we had some back and forth as the intent was to use it as default for QtQuick applications, but that turned out to not be feasible. Instead, we came up with a solution where we use a proxy style as the widget style, which allows Union to be selected from System Settings. For widgets, we forward the styling to the Breeze style still, but when you select the Union style, QtQuick applications will (after restarting the application) use Union for their styling.1
This allows anyone with Plasma 6.7 to opt-in and test how well Union works with their favorite applications, with an easy way to go back should things not work correctly. We already received several bug reports of things not working quite correctly, most of which have been resolved. If you have not yet checked how well Union works for you, give it a try!
Styles and Style Selection
The above solution for selecting Union is mostly intended as an easy way to test Union with Plasma 6.7. It is not meant to be a long-term solution. Instead, we want applications to use Union by default, with a page in System Settings that allows selecting which Union style to use. Initially, that page and the current Application Style will live side-by-side in some form. Once there are a lot more Union styles, we can look at dropping the current Application Style page or moving it somewhere less prominent.
Creating a style selection page required some extra work to make sure that we are able to correctly identify styles and associate metadata with those styles. This work has recently completed, resulting in a fairly big overhaul of the core style loading of Union. Unfortunately, this does mean any styles that have been created so far will need conversion to the new format. The advantage is that now the style determines what input format is needed for the style, which leads to reduced code, increased robustness and easier backward compatibility in the future.
With Union now having a proper definition of what constitutes a "style", it became fairly straightforward to build a System Settings page to allow selecting the active style. The first version is mostly a copy of the current "Application Style" page, but this will evolve over time as that design is not ideal for what Union is capable of. In addition to the System Settings page, there is also a command-line tool to make it easier to work with Union's styles.
Widgets
Over in Qt Widgets land, Akseli Lahtinen has been chipping away at building a proper Qt Widgets style for Union. This is slow going, as there is little documentation on how to actually work with the QStyle API and it is also showing its age.
Still, progress is being made. At this point, the Union widgets style is no longer a proxy style, with everything being rendered by Union itself. Things are starting to look similar to both the Qt Quick side and to the Breeze QStyle. We are unsure if the work will be finished by the time Plasma 6.8 is released, but we are certainly trying to get it there.
Other
There have been several discussions with regards to what capabilities Union needs to have to make it possible to support other styles. This includes discussions around porting Oxygen over to Union to increase the compatibility of Oxygen with Qt Quick applications. There is also ongoing work to build the new Ocean style using Union.
Hopefully with Plasma 6.8 we can move things from being in a tech preview state to become a core part of KDE's experience. But there is still a lot of work ahead of us. If you would like to help get things done, take a look at the Union repository or come talk to us over on Matrix.
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Assuming the application does not hardcode the style to use as several KDE applications did. ↩