fix: update sibling_slot immediately on component reuse

[Madoshakalaka]

Fixes #4092

Impacted usage patterns

The bug requires three children in a sibling list, left to right:

1. A child that transitions from no DOM output to some
2. A component rendering zero DOM nodes
3. A child whose DOM node is removed in the current render

Opposing conditionals with an empty component between them

if *toggled { <span/> }
<Empty/>
if !*toggled { <div>{"old"}</div> }

State-driven show/hide swap

if *state == View::New { <NewPanel/> }
<Empty/>
if *state == View::Old { <OldPanel/> }

Empty list gaining items while a sibling disappears

{ for items.iter().map(|i| html!{ <span>{i}</span> }) }
<Empty/>
if show_footer { <footer/> }

Multiple empty components

if *toggled { <span/> }
<EmptyA/>
<EmptyB/>
if !*toggled { <div>{"old"}</div> }

In all cases, the "empty component" can be anything whose rendered output produces zero DOM nodes: html! {}, html! { if false { <div/> } }, or a component wrapping another empty component.

Checklist

• I have reviewed my own code
• I have added tests

[github-actions]

Benchmark - core

Yew Master

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.126 ns      │ 3.436 ns      │ 2.129 ns      │ 2.235 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.126 ns      │ 3.375 ns      │ 2.129 ns      │ 2.156 ns      │ 100     │ 1000000000

[github-actions]

Visit the preview URL for this PR (updated for commit 3c9adc7):

https://yew-rs-api--pr4093-fix-insert-before-cr-gzekan40.web.app

_{(expires Mon, 06 Apr 2026 11:02:42 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	100.818	100.818	0	0.000%
boids	168.448	168.665	+0.217	+0.129%
communication_child_to_parent	94.076	94.200	+0.124	+0.132%
communication_grandchild_with_grandparent	105.914	106.194	+0.280	+0.265%
communication_grandparent_to_grandchild	102.257	102.539	+0.282	+0.276%
communication_parent_to_child	91.486	91.610	+0.124	+0.136%
contexts	105.977	106.447	+0.471	+0.444%
counter	86.798	86.798	0	0.000%
counter_functional	88.834	88.834	0	0.000%
dyn_create_destroy_apps	90.713	90.713	0	0.000%
file_upload	99.812	99.812	0	0.000%
function_delayed_input	94.807	94.807	0	0.000%
function_memory_game	173.664	174.198	+0.534	+0.308%
function_router	395.375	396.925	+1.550	+0.392%
function_todomvc	164.955	165.362	+0.407	+0.247%
futures	235.551	235.551	0	0.000%
game_of_life	105.099	105.099	0	0.000%
immutable	259.314	259.927	+0.612	+0.236%
inner_html	81.341	81.341	0	0.000%
js_callback	109.957	110.301	+0.344	+0.313%
keyed_list	180.407	180.532	+0.125	+0.069%
mount_point	84.714	84.714	0	0.000%
nested_list	113.660	113.940	+0.280	+0.247%
node_refs	92.087	92.211	+0.124	+0.135%
password_strength	1719.252	1719.376	+0.124	+0.007%
portals	93.560	93.684	+0.124	+0.133%
router	366.017	367.248	+1.231	+0.336%
suspense	113.962	114.433	+0.471	+0.413%
timer	88.943	88.943	0	0.000%
timer_functional	99.368	99.519	+0.150	+0.151%
todomvc	142.661	142.661	0	0.000%
two_apps	86.711	86.711	0	0.000%
web_worker_fib	136.457	136.737	+0.280	+0.205%
web_worker_prime	187.640	187.920	+0.280	+0.149%
webgl	83.486	83.486	0	0.000%

✅ None of the examples has changed their size significantly.

[github-actions]

Benchmark - SSR

Yew Master

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.917	311.643	311.290	0.232
Hello World	10	483.256	517.385	490.767	9.894
Function Router	10	35414.938	39281.304	36807.140	1213.993
Concurrent Task	10	1006.042	1007.536	1006.727	0.549
Many Providers	10	1111.340	1140.981	1124.389	10.814

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.939	311.629	311.206	0.221
Hello World	10	468.545	486.141	473.174	5.645
Function Router	10	33716.661	35806.636	34366.594	671.805
Concurrent Task	10	1005.772	1007.439	1006.825	0.619
Many Providers	10	1099.961	1168.814	1111.735	20.623

[finnbear]

Thanks so much for the quick investigation and fix!

I can confirm this fixes my app:

[patch.crates-io]
yew = { git = "https://github.com/yewstack/yew.git", branch = "fix/insert-before-crash" }