The Linux world[1] has got together and is banging out a solution for a somewhat bigger problem that the multitude of X toolkits and their different (non) approach to HiDPI.
It's called Wayland, it took some time to make and next week, new Fedora is coming out that uses it by default (until now you could "only" opt-in during login).
[1] Except Ubuntu, they have their own NIH solution, of course.
Wayland, the solution where not only does every toolkit have a different and often incomplete approach to HiDPI, they now all have their own implementations of window titlebars and widgets too?
Toolkits always had their own implementation of widgets, that's not even Linux or X11 specific. The same applies to Windows and macOS: ever noticed that MS Office has different widgets than Photoshop? The only difference is, that Windows and macOS had one or more widget libraries supplied with the system and synced look&feel for them, while there was no canonical widget library or l&f for X11.
Title bars are up to compositor (it was up to window manager in X11). Even client side window decorations (i.e. mostly title bar combined with toolbar) are nothing new, if that is what you mean with custom implementation of window titlebars.
It's called Wayland, it took some time to make and next week, new Fedora is coming out that uses it by default (until now you could "only" opt-in during login).
[1] Except Ubuntu, they have their own NIH solution, of course.