diff --git a/reacton/core.py b/reacton/core.py
index e35d171..4211e13 100644
--- a/reacton/core.py
+++ b/reacton/core.py
@@ -2489,6 +2489,10 @@ class _RenderContextFast(_RenderContext):
     # without walking either.
     # ------------------------------------------------------------------
 
+    # > 0 while the render phase walks the new children of a widget that replaces a
+    # widget of another type (see _render)
+    _replacing = 0
+
     def _set_rerender_needed(self, reason: str):
         self._rerender_needed_reasons.append(RerenderReason(reason=reason))
         self._rerender_needed = True
@@ -2537,7 +2541,18 @@ def _render(self, element: Element, default_key: str, parent_key: str):
                 del context.children_next[key]
             # the element arguments are part of this component's element tree
             if el.kwargs:
-                self._visit_children(el, key, parent_key, self._render)
+                el_reconciled = context.elements.get(key)
+                if el_reconciled is not None and el_reconciled.component != el.component:
+                    # reconciliation replaces the widget at this key, and first removes the
+                    # old subtree, including the component contexts in it: the walk below
+                    # must not keep one of those as it is (see the fast path further down)
+                    self._replacing += 1
+                    try:
+                        self._visit_children(el, key, parent_key, self._render)
+                    finally:
+                        self._replacing -= 1
+                else:
+                    self._visit_children(el, key, parent_key, self._render)
             return
 
         assert isinstance(el.component, ComponentFunction)
@@ -2555,6 +2570,7 @@ def _render(self, element: Element, default_key: str, parent_key: str):
             not self._walk_all
             and el is el_prev
             and not el.is_shared
+            and not self._replacing
             and context_previous is not None
             and context.children.get(key) is context_previous
             and not context_previous.needs_render
diff --git a/reacton/core_test.py b/reacton/core_test.py
index 21f3784..68b2bed 100644
--- a/reacton/core_test.py
+++ b/reacton/core_test.py
@@ -3794,3 +3794,28 @@ def App():
     set_offset(0)
     assert [child.description for child in vbox.children[1:]] == ["0", "1"]
     rc.close()
+
+
+def test_replace_parent_same_child_element():
+    # like test_replace_parent, but the child element is the same object in every render
+    # (it comes from outside): replacing its parent widget removes the child's subtree,
+    # so the child cannot keep its previous widget
+    set_vertical = lambda x: None  # noqa
+    child = ButtonComponentFunction(description="Hi")
+
+    @react.component
+    def Test(child):
+        nonlocal set_vertical
+        vertical, set_vertical = react.use_state(True)
+        Container = w.VBox if vertical else w.HBox
+        with w.VBox() as main:
+            Container(children=[child])
+        return main
+
+    box, rc = react.render(Test(child), handle_error=False)
+    assert len(rc.find(widgets.Button)) == 1
+    set_vertical(False)
+    assert len(rc.find(widgets.HBox).find(widgets.Button)) == 1
+    set_vertical(True)
+    assert len(rc.find(widgets.Button)) == 1
+    rc.close()
diff --git a/reacton/fuzz_test.py b/reacton/fuzz_test.py
new file mode 100644
index 0000000..a10f8d5
--- /dev/null
+++ b/reacton/fuzz_test.py
@@ -0,0 +1,160 @@
+# Compare the fast renderer (REACTON_FAST=1) with the default renderer on random component
+# trees: the fast renderer skips parts of the tree that did not change, and after every
+# (batch of) state change(s) it must give the same widgets and run the same effects, in the
+# same order, as the default renderer that walks everything.
+import os
+import random
+import unittest.mock
+import zlib
+from typing import Any, Callable, Dict, List, Optional
+
+import ipywidgets as widgets
+import pytest
+
+import reacton
+import reacton as react
+
+from . import core
+from . import ipywidgets as w
+from .core_test import cleanup_guard  # noqa: F401  (autouse: no leaked widgets or callbacks)
+
+
+def _random_app(registry, log):
+    # component trees that change shape with their state (container type flips, a changing
+    # number of children, keys, shuffles, fragments, components whose root is a component,
+    # leaves whose root widget changes type, effects that set state, caught exceptions)
+    def h(*args):
+        # not hash(): the hash of a tuple differs between Python versions, and every version
+        # must generate the same trees
+        return zlib.crc32(repr(args).encode())
+
+    @react.component
+    def Leaf(id):
+        value, set_value = react.use_state(0)
+        registry[id] = set_value
+        if value == 0 and h(id) % 5 == 0:
+            # state set during the first render (a second render pass)
+            set_value(1)
+        kind = h(id, value) % 4
+        if kind == 0:
+            return w.Label(value=f"leaf {id} {value}")
+        if kind == 1:
+            return reacton.Fragment(children=[w.Button(description=f"f{id}.{i}") for i in range(value % 3)])
+        return w.Button(description=f"leaf {id} {value}")
+
+    @react.component
+    def Wrapper(id):
+        return Leaf(id * 7 + 1)
+
+    @react.component
+    def Thrower(id):
+        value, set_value = react.use_state(0)
+        registry[id] = set_value
+        if value == 7 or (value == 0 and h(id) % 3 == 0):
+            # also raises in its first render (in a new subtree)
+            raise ValueError(f"boom {id}")
+        return w.Button(description=f"thrower {id} {value}")
+
+    @react.component
+    def Catcher(id):
+        exception, clear = react.use_exception()
+        state, set_state = react.use_state(0)
+
+        def set_value(value):
+            clear()
+            set_state(value)
+
+        registry[id] = set_value
+        if exception:
+            return w.Label(value=f"caught {exception}")
+        return w.HBox(children=[Thrower(id * 3 + 2), Leaf(id * 3 + 1)])
+
+    @react.component
+    def Node(id, depth):
+        state, set_state = react.use_state(0)
+        registry[id] = set_state
+        seed = h(id, state)
+        rnd = random.Random(seed)
+
+        def effect():
+            log.append(("effect", id, state))
+            if state % 5 == 4:
+                set_state(state + 1)
+
+        react.use_effect(effect, [state])
+        children: List[Any] = [w.Label(value=f"node {id} {state}")]
+        for i in range(rnd.randint(0, 4)):
+            child_id = id * 10 + i
+            r = rnd.random()
+            if depth < 3 and r < 0.4:
+                child = Node(child_id, depth + 1)
+            elif r < 0.6:
+                child = Wrapper(child_id)
+            elif r < 0.7:
+                child = Catcher(child_id)
+            else:
+                child = Leaf(child_id)
+            if rnd.random() < 0.3:
+                child = child.key(f"k{child_id}")
+            children.append(child)
+        if rnd.random() < 0.2:
+            rnd.shuffle(children)
+        if depth == 0:
+            # the root keeps its widget type: without a container, reacton does not support a
+            # root component whose widget changes
+            return w.VBox(children=children)
+        if seed % 7 == 0:
+            return reacton.Fragment(children=children)
+        return (w.VBox if seed % 3 else w.HBox)(children=children)
+
+    return Node
+
+
+def _widget_signature(widget):
+    # a closed widget still has its traits: compare that it is open too
+    closed = widget.comm is None
+    if isinstance(widget, widgets.Box):
+        return (type(widget).__name__, closed, [_widget_signature(child) for child in widget.children])
+    return (type(widget).__name__, closed, getattr(widget, "value", None), getattr(widget, "description", None))
+
+
+def _run_random_updates(fast: bool, seed: int, steps: int, batches: Optional[List] = None):
+    registry: Dict[int, Callable] = {}
+    log: List[tuple] = []
+    Node = _random_app(registry, log)
+    record = batches is None
+    batches = [] if batches is None else batches
+    choices = random.Random(seed)
+    results = []
+    with unittest.mock.patch.dict(os.environ, {"REACTON_FAST": "1" if fast else "0"}):
+        widget, rc = react.render_fixed(Node(1, 0), handle_error=False)
+        assert isinstance(rc, core._RenderContextFast) == fast
+        for step in range(steps):
+            if record:
+                ids = sorted(registry)
+                batches.append([(choices.choice(ids), choices.randint(0, 9)) for _ in range(choices.choice([1, 1, 1, 2, 3]))])
+            batch = batches[step]
+            log.clear()
+            with rc:
+                for id, value in batch:
+                    if id in registry:
+                        registry[id](value)
+            results.append((_widget_signature(rc.last_root_widget), sorted(registry), list(log)))
+        rc.close()
+    return results, batches
+
+
+@pytest.mark.parametrize("seed", range(12))
+def test_renderers_agree_on_random_updates(seed):
+    # the fast renderer skips (and only partially walks) parts of the tree: after every
+    # (batch of) state change(s) it must give the same widgets, and run the same effects in
+    # the same order, as the default renderer that walks everything
+    level = core.logger.level
+    core.logger.setLevel(core.logging.CRITICAL)  # the thrower logs tracebacks
+    try:
+        expected, batches = _run_random_updates(False, seed, 25)
+        got, _ = _run_random_updates(True, seed, 25, batches)
+    finally:
+        core.logger.setLevel(level)
+    for step, (a, b) in enumerate(zip(expected, got)):
+        assert a == b, f"step {step}, batch {batches[step]}"