Back in 2017, a bug report was created on bugzilla asking for ‘Locally Integrated Menus’ like the Unity desktop. This was a feature where the menubar of an app was displayed in the titlebar, appearing on hover by default (though you could make it always visible).
Over the next couple years there was some development, but it was mostly in individual window decorations such as “Material Decoration”. In 2021, there was a merge request made to finally add LIMs into KDE plasma as an option for titlebars. Unfortunately, due to proximity to the release of plasma 6, reliance on x11, and “a technical disagreement over where it should live” [Guido Iodice, @giodice, 2023 comment under merge request], the merge request has had no changes since August 11, 2021.
Personally, I would love to have this feature as it would save a entire menubar’s worth of vertical space on my screen and would allow me to make use of some of the dead space in my titlebars. similar sentiments were expressed throughout the threads under both the bug report and the merge request. many people also talked about giving the option of showing on hover (like unity) or showing always (my preference), and some even suggested making it the default behavior. Do you think this would be a good feature?
from my reading of it, it seems like the thing that kept it from being merged 3 years ago was some disagreement on what system component should be in charge of it (at the time it worked) and what is keeping it now is that it was made for plasma 5.9 (putting it some 700 or so commits behind where it needs to be).
also, I don’t really see how this would be any more “client side” than the hamburger menu titlebar button already is. it’s just a longer button (or set of buttons, technically), however long it needs to be for the number of options it has.