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.
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.