At my work (C++/Qt) we do a bunch of GUIs for a set of several customer applications that need the same look and feel. So we do end up reusing the team's custom

At my work (C++/Qt) we do a bunch of GUIs for a set of several customer applications that need the same look and feel. So we do end up reusing the team's custom widgets wherever they answer our requirements. I find I often write something as helper functions for a GUI or library, then break it out when someone else needs to call it. It's definitely better from my perspective to leave it sitting in the old class until you come up with something more urgent than scenarios for how someone might use such a library someday. Coding standards are also extremely important to ensure that someone will be able to look at your code and use it without deciding to rewrite it from scratch.