What is a RenderObject in Flutter?

A RenderObject is the core low-level object in Flutter's rendering pipeline responsible for layout and painting. It receives BoxConstraints from its parent, calculates its own size, positions its children, and paints to the Canvas. Most developers never interact with RenderObjects directly — they use Widgets, which create and manage RenderObjects internally. Custom RenderObjects are created when building highly optimized custom layouts or widgets that cannot be achieved by composing existing widgets.