Union in Plasma 6.7

[Akseli]

On 15 May 2026 13.16.05 EEST, Nicolas Fella <nicolas.fella at gmx.de> wrote:
>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
>

I think this can be fixed, make it case insensitive.

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

Why are apps doing this?

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

For this I have seen couple MRs in Union already.

>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
>

I'd like us to wait before jumping. I believe these issues can be fixed.