Union in Plasma 6.7

[Nicolas Fella]

On 5/1/26 12:42 PM, Arjen Hiemstra wrote:
> Hi,
>
> With Plasma 6.7 we will have a release of Union, the new style engine that
> we've been working on for a while now. The current state is that we have a
> fully implemented QtQuick Controls style along with a Breeze reimplementation
> in CSS. Some of us have been daily driving it for a while now and while there
> are regressions, most of these are to the extent of slightly different paddings
> or mismatching colors.
>
> However, we cannot test all applications out there on our own, so we may have
> missed things. We really need more testing to figure out if there is major
> breakage and if so, how things break.
>
> This isn't really possible without a more general rollout of Union, so we had
> some discussion yesterday about what exactly we want to do for 6.7. I think
> there have been three options floated about:
>
> - We switch the default QtQuick Controls style. Since only Breeze is currently
> implemented in Union, switching it without some fallback would annoy people
> who don't use Breeze QtWidgets, so the current idea is to only use it when the
> user has selected Breeze. The main drawback is that there's no simple escape
> hatch for if an application does break to such and extent that it is unusable.
>
> - We add an option somewhere for users to opt in using Union. The advantage is
> that this could be fully opt-in. It could also explain more about what it is
> and what the state of things is in UI. The disadvantage is that for most users
> it is going to be a meaningless option that they won't fully know what the
> implications are, since it's mainly a technical option.
>
> - We do not do anything and rely on users setting `QT_QUICK_CONTROLS_STYLE` to
> Union. This I think is the worst outcome, as it makes it quite unlikely that
> many people use it, which means we don't get the testing coverage for Union
> that we need at this point.
>
> I know this discussion should have been started sooner, but the soft freeze
> date kinda snuck up on me, so here we are.

Hi,

sorry to be the party pooper here, but the whole rollout isn't going well:

- The logic in 
https://invent.kde.org/plasma/plasma-integration/-/merge_requests/218 is 
flawed, Breeze needs to be Uppercase for it to trigger

- Most of our QML apps have QQuickStyle::setStyle("org.kde.desktop") in 
their main.cpp, overriding the default from plasma-integration

- Distros are struggling to package Union due to Rust issues in 
cxx-rust-cssparser

Given that this prevents useful testing during the beta we can't ship 
anything like this. Therefore 
https://invent.kde.org/plasma/plasma-integration/-/merge_requests/223

Please try again for 6.8

Cheers

Nico