The interface after the mockup.
The design is not finished when the frame looks right. It is finished when the system survives reality.
A static frame is a proposal. The actual interface begins when content changes length, data arrives late, focus moves by keyboard and the viewport refuses to match the presentation.
Front-end knowledge changes the decisions made before implementation. It exposes the difference between a visual effect and an interaction model, between a flexible system and a collection of carefully staged screenshots.
Staying close to the code is not about controlling every implementation detail. It is about protecting the hierarchy, pace and behaviour that made the direction worth building.
The handoff is not a border. It is where a product either keeps its point of view or slowly exchanges it for defaults.