clone() build() onTap index mutate stretch INCOMING_LIST LOCAL_STATE TIMELINE_STEP INTRINSIC_HEIGHT USER_TAP TOGGLE_EXPANSION Immutable list of events passed down to the widget. Cloned inside TimelineWidget to allow local mutation. Builds the step content. Row height changes if expanded. Forces the connector line to stretch to the tallest card. User interacts with a specific rendered step card. Calls setState() to update the specific index in _items.