KDE Review: Union and cxx-rust-cssparser

[Arjen Hiemstra]

On Thursday, 26 February 2026 11:07:28 Central European Summer Time Arjen 
Hiemstra wrote:
> Hello all,

Hi,

It's been a while since I sent the original review request and in the meantime 
we've finished with all the review tasks. We would like to include Union in the 
upcoming Plasma 6.7 release. We've been daily driving it for a while now and 
while there's some issues here and there, overall it's almost 
indistinguishable from qqc2-desktop-style using Breeze QStyle.

- Arjen


> 
> I'd like to start the review process for two projects, Union and its
> dependency cxx-rust-cssparser. The relevant review issues are https://
> invent.kde.org/plasma/union/-/issues/115 for Union and
> https://invent.kde.org/ libraries/cxx-rust-cssparser/-/issues/3 for
> cxx-rust-cssparser.
> 
> Union is a new style engine that intends to unify styling between the
> various styling implementations we currently have, covering both QtQuick
> and QtWidgets. It uses CSS as input format, though it is designed to allow
> multiple input formats. The CSS input is powered by cxx-rust-cssparser
> which is a thin C++ wrapper around a rust-based CSS parser built upon the
> rust "cssparser" crate.
> 
> The current state is that we have a near-complete implementation of a
> QtQuick style, along with a fairly complete implementation of Breeze in
> CSS. There is also Kirigami integration on the QtQuick side. The QtWidgets
> side is still rather limited, we can only style buttons at the moment.
> There will be more progress there once we're finished with the QtQuick
> side.
> 
> The plan is to release Union along with Plasma 6.7, at the very least as a
> "tech preview", but hopefully in such a state that we can start replacing
> our other QtQuick styles. After a few releases I hope to make it a
> framework that replaces all our styles, including the QtWidgets side.
> 
> To get to that state we'll need more testing however. If you want to help
> test, running a QtQuick application with `QT_QUICK_STYLE=org.kde.union`
> after building should run it with Union's QtQuick style. If you find any
> issues, do let us know.
> 
> - Arjen