feat: out-of-order streaming SSR for Suspense

[Madoshakalaka]

Description

We now stream suspense fallback UI immediately during SSR. So long running data-fetching won't bottleneck the page.

#[component]
fn ProductPage() -> HtmlResult {
    let data = use_prepared_state!(
        (),
        async move |_| -> String { fetch_from_database().await } // this takes 5 whopping seconds
    )?.unwrap();

    Ok(html! { <div>{data}</div> })
}

#[component]
fn App() -> Html {
    html! {
        <>
            <h1>{"My Store"}</h1>
            <Suspense fallback={html! { <p>{"Loading..."}</p> }}>
                <ProductPage />
            </Suspense>
            <footer>{"Footer streams immediately"}</footer>
        </>
    }
}

Before: the entire response blocks until fetch_from_database() completes.
After: <h1>, the fallback <p>Loading...</p>, and <footer> stream immediately. The resolved <div> swaps in when the fetch finishes.

Resolves #3619

Checklist

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

[github-actions]

Visit the preview URL for this PR (updated for commit 45a31f1):

https://yew-rs--pr4031-feat-ssr-out-of-orde-n7rapk68.web.app

_{(expires Mon, 09 Mar 2026 12:58:46 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[github-actions]

Benchmark - core

Yew Master

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.403 ns      │ 3.005 ns      │ 2.406 ns      │ 2.416 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.361 ns      │ 3.629 ns      │ 2.375 ns      │ 2.938 ns      │ 100     │ 1000000000

[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	100.188	100.188	0	0.000%
boids	168.073	168.073	0	0.000%
communication_child_to_parent	93.464	93.464	0	0.000%
communication_grandchild_with_grandparent	105.240	105.240	0	0.000%
communication_grandparent_to_grandchild	101.603	101.603	0	0.000%
communication_parent_to_child	90.892	90.892	0	0.000%
contexts	105.142	105.142	0	0.000%
counter	86.274	86.274	0	0.000%
counter_functional	88.269	88.269	0	0.000%
dyn_create_destroy_apps	90.312	90.312	0	0.000%
file_upload	99.335	99.335	0	0.000%
function_delayed_input	94.362	94.362	0	0.000%
function_memory_game	172.937	172.937	0	0.000%
function_router	406.649	406.649	0	0.000%
function_todomvc	164.156	164.156	0	0.000%
futures	235.151	235.151	0	0.000%
game_of_life	104.709	104.709	0	0.000%
immutable	255.739	255.739	0	0.000%
inner_html	80.795	80.795	0	0.000%
js_callback	109.369	109.369	0	0.000%
keyed_list	179.717	179.717	0	0.000%
mount_point	84.139	84.139	0	0.000%
nested_list	113.048	113.048	0	0.000%
node_refs	91.518	91.518	0	0.000%
password_strength	1729.144	1729.144	0	0.000%
portals	93.027	93.027	0	0.000%
router	377.198	377.198	0	0.000%
suspense	113.450	113.450	0	0.000%
suspense_ssr	N/A	219.973	N/A	N/A
timer	88.625	88.625	0	0.000%
timer_functional	98.867	98.867	0	0.000%
todomvc	142.080	142.080	0	0.000%
two_apps	86.139	86.139	0	0.000%
web_worker_fib	136.215	136.215	0	0.000%
web_worker_prime	187.452	187.452	0	0.000%
webgl	83.217	83.217	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	310.550	314.212	311.216	1.117
Hello World	10	481.108	484.256	482.264	1.020
Function Router	10	31270.077	31870.057	31546.526	158.963
Concurrent Task	10	1005.690	1007.330	1006.703	0.523
Many Providers	10	1069.457	1109.531	1084.985	10.312

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	291.082	291.668	291.379	0.162
Hello World	10	483.369	531.510	493.834	15.461
Function Router	10	31653.098	32351.982	31944.797	228.698
Concurrent Task	10	1004.829	1006.892	1006.493	0.632
Many Providers	10	1065.969	1117.188	1085.643	16.333