Union in Plasma 6.7

[Akseli]

On 1 May 2026 13.42.23 EEST, Arjen Hiemstra <ahiemstra at heimr.nl> 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.
>

I suppose this would be the safest bet, but I would make it opt-out. We should trust our new defaults, but in case someone has issues with it, they can toggle it off. 
We should also show a popup asking the user to report any issues as why they decided to toggle it off, with link to our bug tracker.



>- 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.
>
>- Arjen
>
>