Union in Plasma 6.7

[Nicolas Fella]

On 5/16/26 2:21 PM, Akseli wrote:
> 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.
I'm sure they can be fixed. The point is: We wanted to test it in the 
beta, but now we shipped beta without it enabled. Because of that we 
won't get the testing we wanted, so we can't enable it in 6.7