Survive a widget/component slot flip inside one render call

[maartenbreddels]

Why

Seen in production through solara's RoutingProvider: its get_nav_widget effect raised
KeyError: 'Element VBox(children = ...) was found to be in a previous render, you may have used a stale element'
and the whole app was replaced by the traceback. It took two reacton defects in a row.

1. A render call with a nested pass, where a positional slot holds a widget element in the first pass and a function component in the nested pass, while that component still has its context from the last committed render.
   _render compared the component element against the widget element left in elements_next and tripped assert not isinstance(el_prev.component, ComponentWidget).
   The state is reachable from ordinary app code: a write from another thread lands while a render is in flight, reacton defers it into a nested pass, and a conditional sibling that appears in one pass shifts the slots.
2. That assert, like any exception from reacton's own bookkeeping inside _render, escaped render() and left the tree half updated.
   Components that re-executed in the aborted pass had already chained a new effect closure (previous_effect.next) over elements of that pass, but their root_element_next was never set.
   The next render that did not re-execute such a component ran the stale closure, and get_widget failed on an element that was never reconciled.

What

• _render, both renderers: a widget el_prev from an earlier pass of the same call means "render this component", not an assert. Nothing was committed between the passes, so the component was never unmounted and keeps its state.
• render(): an exception that escapes a render pass now discards what the pass staged (effect.next, root_element_next, elements_next, collected exceptions) and marks every context dirty, so the next render starts from the last committed state. children_next is kept, it also holds contexts pre-created by state_set(). Aborts inside reconciliation are left alone: widgets created halfway need the normal removal path.

Tests

Four new tests in core_test.py, all fail on master for the right reason and pass with the fix, on the classic and the REACTON_FAST renderer:

• the slot flip must render and keep the component's state
• the flip must not strand the chained get_widget effect of a parent that re-executed in the aborted pass (the production chain)
• a duplicate key aborting in the first pass, or in a nested pass, must not strand a chained effect either

The cost of the discard is one extra full render after an already-failed render.

🤖 Generated with Claude Code