This suggestion kicked off after recently dealing with Client-Side-Decorations in GTK3 applications and having to install a patch called GTK3-noCSD. It's additionally relevant now that KDE just implemented what it calls the hamburger menu, which seems like a step in the right direction.

I feel it's legitimate to ask whether the time has come to support applications being allowed to draw menu / tab / toolbar items directly in the KWin titlebar. This is one of the few features of modern desktops KDE doesn't yet offer, with Gnome being a common example of it: It allows applications to draw special items right in place of the window title, which can host tabs or search fields or backward / forward buttons or anything else. I'd also reference the many Chromium / Firefox KDE users who've been dreaming for years about having the ability to make their browser tabs show up in the window titlebar for extra space.

https://i.imgur.com/M2LlXiB.png

I'm not sure how such a system would work under the hood, though I do know some of the basics have already been done; We have the Application Menu titlebar button, which allows putting the menu bar of Qt and GTK applications in a button or even a plasma widget... we could thus start by offering the option to draw the titlebar on the KWin decoration. If we're talking more complex widgets then it may be a different story: Applications need a way to tell KWin they want it to draw things like a search field or custom button in the titlebar, which may require changes to how Qt and GTK decorations communicate to window managers to request drawing application content. Such a framework should already exist: I'd once more point to the Firefox browser which knows to tell a compatible window manager that it wants its tabs drawn in the title bar.