What is the Element tree in Flutter and what role does it play?

The Element tree is Flutter's internal, mutable representation of the widget tree that persists across rebuilds. While widgets are recreated on every build() call, elements are long-lived — they hold the actual state of StatefulWidgets and connect widgets to their corresponding RenderObjects. When Flutter rebuilds, it performs a reconciliation: it compares the new widget configuration against the existing element. If the widget type and key match, the element is updated in place (much cheaper than destroying and recreating it). This is why Flutter can be fast despite frequently rebuilding widgets.