Depending on widgets external to the toolkit is a no-go for me. It unnecessarily bloats dependencies and there’s a high risk of the widget eventually being abandoned.

I totally get your sentiment and also try to instill the culture of "add dependency only if it's absolutely not possible to avoid it". For small widgets, I often just copy the code into my tree. And I absolutely hate when library authors break API for no reason (something that the Dart ecosystem inherited from the web one).

But limiting yourself only to standard widgets seems to be extreme.

It’s not that I limit myself to standard widgets, it’s more that the toolkits I use have a full enough complement of standard widgets that any custom needs are easily met by tweaking one of those.

In UIKit for instance I typically build custom buttons by subclassing UIControl, which gives a nice “blank slate” control that has all the basics of interaction covered without the specifics of UIButton. It’s easy to build a custom button that’s as well-behaved as a standard UIButton that way.