Definitions are complicated, it feels like they should be static when the definition of what a front-end developer does is always changing as technology comes up with different mediums like smartphones, watches, VR, etc.
I've thought about this a lot and I would say something like this:
A front-end/UI designer is concerned about the _user_ interface with the application, that is, how a user interacts and utilizes an application.
A front-end developer is concerned about the _client_ interface with the application, such as what browsers support what and how you can work with that, screen sizes and how to best utilize them(can't have a full word processor and keyboard in a smartwatch), how do you flow data through the client in a way that it doesn't hog the system's memory or makes everything slow, tradeoffs between server-side and client-side rendering, caching, optimization, optimistic UI, unstable connections, progressive enhancement and graceful degradation, and much more.
Of course, this doesn't mean a front-end developer/designer won't have to dabble in both roles eventually.
I've thought about this a lot and I would say something like this:
A front-end/UI designer is concerned about the _user_ interface with the application, that is, how a user interacts and utilizes an application.
A front-end developer is concerned about the _client_ interface with the application, such as what browsers support what and how you can work with that, screen sizes and how to best utilize them(can't have a full word processor and keyboard in a smartwatch), how do you flow data through the client in a way that it doesn't hog the system's memory or makes everything slow, tradeoffs between server-side and client-side rendering, caching, optimization, optimistic UI, unstable connections, progressive enhancement and graceful degradation, and much more.
Of course, this doesn't mean a front-end developer/designer won't have to dabble in both roles eventually.