fix: move deferred rendered off the scheduler onto BaseSuspense

The previous approach routed a resuming child's rendered through the
scheduler's `rendered` queue, which drains only after the update+render
queues. That worked for a single suspender but not for siblings: if A
resumed while B was still pending, A's rendered still fired before the
boundary un-suspended, because Suspense's render with suspended=true
still doesn't shift children into the live tree.

Hand the pending rendered directly to the ancestor BaseSuspense as a
PendingRendered, keyed by comp_id in a Vec. BaseSuspense drains
this list in its own rendered lifecycle, gated on
self.suspensions.is_empty() - i.e. the commit on which reconcile has
already shifted children from the detached parent into the live tree.
Re-committed children absorb into the existing entry so
first_render=true survives a suspend/resume/suspend/resume cycle.

Author: Madoshakalaka

packages/yew/src/html/component/lifecycle.rs

@@ -510,20 +510,28 @@ impl ComponentState {
                 let first_render = !self.has_rendered;
                 self.has_rendered = true;
 
-                // When resuming from suspension, the component's DOM nodes live in a
-                // detached parent. The Suspense ancestor must process the Resume
-                // message and re-render to move children into the live DOM before
-                // effects run. Use the `rendered` queue (not `rendered_first`) so
-                // the scheduler processes the Suspense update and render first.
-                let schedule_as_first = first_render && !resuming_from_suspension;
-                scheduler::push_component_rendered(
-                    self.comp_id,
-                    Box::new(RenderedRunner {
-                        state: shared_state.clone(),
+                if resuming_from_suspension {
+                    // The DOM we just reconciled still lives in the ancestor
+                    // Suspense's detached parent. The Suspense must process
+                    // the Resume message and re-render to shift children into
+                    // the live tree before our effects observe the DOM.
+                    // Hand the pending `rendered` to the Suspense; it will
+                    // re-schedule it once it fully un-suspends.
+                    let pending = PendingRendered::new(shared_state.clone(), first_render);
+                    let suspense_scope = scope
+                        .find_parent_scope::<BaseSuspense>()
+                        .expect("a resuming component must have a Suspense ancestor");
+                    BaseSuspense::defer_rendered(&suspense_scope, self.comp_id, pending);
+                } else {
+                    scheduler::push_component_rendered(
+                        self.comp_id,
+                        Box::new(RenderedRunner {
+                            state: shared_state.clone(),
+                            first_render,
+                        }),
                         first_render,
-                    }),
-                    schedule_as_first,
-                );
+                    );
+                }
             }
 
             #[cfg(feature = "hydration")]
@@ -709,6 +717,47 @@ mod feat_csr {
         pub first_render: bool,
     }
 
+    /// A `rendered` lifecycle deferred by the ancestor `<Suspense>` so it fires
+    /// only after Suspense un-suspends and children's DOM has been shifted into
+    /// the live tree. See `BaseSuspense::defer_rendered`.
+    pub(crate) struct PendingRendered {
+        pub state: Shared<Option<ComponentState>>,
+        pub first_render: bool,
+    }
+
+    impl PendingRendered {
+        pub(crate) fn new(state: Shared<Option<ComponentState>>, first_render: bool) -> Self {
+            Self {
+                state,
+                first_render,
+            }
+        }
+
+        /// Absorb a later-committed pending rendered for the same component:
+        /// keep the latest state but preserve `first_render=true` if either
+        /// side carried it.
+        pub(crate) fn absorb(&mut self, later: Self) {
+            self.state = later.state;
+            self.first_render |= later.first_render;
+        }
+
+        /// Push this onto the scheduler's `rendered` queue.
+        pub(crate) fn schedule(self, comp_id: usize) {
+            let PendingRendered {
+                state,
+                first_render,
+            } = self;
+            scheduler::push_component_rendered(
+                comp_id,
+                Box::new(RenderedRunner {
+                    state,
+                    first_render,
+                }),
+                false,
+            );
+        }
+    }
+
     impl ComponentState {
         #[tracing::instrument(
             level = tracing::Level::DEBUG,
@@ -749,7 +798,7 @@ mod feat_csr {
 }
 
 #[cfg(feature = "csr")]
-pub(super) use feat_csr::*;
+pub(crate) use feat_csr::*;
 
 #[cfg(all(target_arch = "wasm32", not(target_os = "wasi")))]
 #[cfg(test)]

packages/yew/src/html/component/mod.rs

@@ -10,6 +10,8 @@ mod scope;
 use std::rc::Rc;
 
 pub use children::*;
+#[cfg(feature = "csr")]
+pub(crate) use lifecycle::PendingRendered;
 pub use marker::*;
 pub use properties::*;
 #[cfg(feature = "csr")]

packages/yew/src/suspense/component.rs

@@ -14,7 +14,12 @@ pub struct SuspenseProps {
 
 #[cfg(any(feature = "csr", feature = "ssr"))]
 mod feat_csr_ssr {
+    #[cfg(feature = "csr")]
+    use std::cell::RefCell;
+
     use super::*;
+    #[cfg(feature = "csr")]
+    use crate::html::PendingRendered;
     use crate::html::{Component, Context, Html, Scope};
     use crate::suspense::Suspension;
     #[cfg(feature = "hydration")]
@@ -35,11 +40,29 @@ mod feat_csr_ssr {
         Resume(Suspension),
     }
 
-    #[derive(Debug)]
     pub(crate) struct BaseSuspense {
         suspensions: Vec<Suspension>,
         #[cfg(feature = "hydration")]
         hydration_handle: Option<SuspensionHandle>,
+        /// Rendered runners for child components that resumed while this
+        /// Suspense was still suspended (because of other pending siblings).
+        /// Drained in `rendered` once the Suspense fully un-suspends, so
+        /// effects fire only after children's DOM has been shifted into the
+        /// live tree.
+        ///
+        /// A small `Vec` is used over a map; the expected population is the
+        /// number of suspending direct descendants resumed in one boundary
+        /// transition, typically just a handful.
+        #[cfg(feature = "csr")]
+        pending_rendered: RefCell<Vec<(usize, PendingRendered)>>,
+    }
+
+    impl std::fmt::Debug for BaseSuspense {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            f.debug_struct("BaseSuspense")
+                .field("suspensions", &self.suspensions)
+                .finish()
+        }
     }
 
     impl Component for BaseSuspense {
@@ -73,6 +96,8 @@ mod feat_csr_ssr {
                 suspensions,
                 #[cfg(feature = "hydration")]
                 hydration_handle,
+                #[cfg(feature = "csr")]
+                pending_rendered: RefCell::new(Vec::new()),
             }
         }
 
@@ -128,13 +153,25 @@ mod feat_csr_ssr {
             }
         }
 
-        #[cfg(feature = "hydration")]
         fn rendered(&mut self, _ctx: &Context<Self>, first_render: bool) {
+            #[cfg(not(feature = "hydration"))]
+            let _ = first_render;
+            #[cfg(feature = "hydration")]
             if first_render {
                 if let Some(m) = self.hydration_handle.take() {
                     m.resume();
                 }
             }
+            // Fire deferred rendered callbacks for children that resumed while
+            // we were still suspended. Only safe now that we're un-suspended:
+            // the last reconcile shifted their DOM into the live tree.
+            #[cfg(feature = "csr")]
+            if self.suspensions.is_empty() {
+                let pending = std::mem::take(&mut *self.pending_rendered.borrow_mut());
+                for (comp_id, p) in pending {
+                    p.schedule(comp_id);
+                }
+            }
         }
     }
 
@@ -146,6 +183,28 @@ mod feat_csr_ssr {
         pub(crate) fn resume(scope: &Scope<Self>, s: Suspension) {
             scope.send_message(BaseSuspenseMsg::Resume(s));
         }
+
+        /// Queue a child component's `rendered` lifecycle to be scheduled once
+        /// this Suspense fully un-suspends and its reconcile has shifted the
+        /// child's DOM into the live tree. If the child already has a pending
+        /// entry (e.g. it re-committed between suspensions), the two are
+        /// merged so `first_render=true` is not lost.
+        #[cfg(feature = "csr")]
+        pub(crate) fn defer_rendered(
+            scope: &Scope<Self>,
+            comp_id: usize,
+            pending: PendingRendered,
+        ) {
+            let Some(comp) = scope.get_component() else {
+                return;
+            };
+            let mut q = comp.pending_rendered.borrow_mut();
+            if let Some(slot) = q.iter_mut().find(|(id, _)| *id == comp_id) {
+                slot.1.absorb(pending);
+            } else {
+                q.push((comp_id, pending));
+            }
+        }
     }
 
     /// Suspend rendering and show a fallback UI until the underlying task completes.

packages/yew/tests/suspense.rs

@@ -899,3 +899,86 @@ async fn use_effect_can_access_dom_after_use_future_resolves() {
         "use_effect should see the rendered DOM element after use_future resolves"
     );
 }
+
+// Companion to #3780: when multiple siblings under the same <Suspense> each
+// suspend independently and resume at different times, every child's effect
+// must see the DOM in the live tree, not the detached parent where Suspense
+// parks children while at least one of them is still pending.
+#[wasm_bindgen_test]
+async fn sibling_suspensions_effects_see_dom() {
+    #[derive(Properties, Clone)]
+    struct ChildProps {
+        id: &'static str,
+        observed: Rc<RefCell<Vec<(&'static str, bool)>>>,
+    }
+
+    impl PartialEq for ChildProps {
+        fn eq(&self, other: &Self) -> bool {
+            self.id == other.id
+        }
+    }
+
+    #[component(Child)]
+    fn child(props: &ChildProps) -> HtmlResult {
+        use_future(|| async {
+            sleep(Duration::ZERO).await;
+        })?;
+
+        let id = props.id;
+        let observed = props.observed.clone();
+        use_effect_with((), move |_| {
+            let found = gloo::utils::document().get_element_by_id(id).is_some();
+            observed.borrow_mut().push((id, found));
+            || {}
+        });
+
+        Ok(html! { <div id={id}></div> })
+    }
+
+    #[derive(Properties, Clone)]
+    struct AppProps {
+        observed: Rc<RefCell<Vec<(&'static str, bool)>>>,
+    }
+
+    impl PartialEq for AppProps {
+        fn eq(&self, _other: &Self) -> bool {
+            true
+        }
+    }
+
+    #[component(App)]
+    fn app(props: &AppProps) -> Html {
+        html! {
+            <Suspense fallback={html! { <div>{"loading"}</div> }}>
+                <Child id="sib-a" observed={props.observed.clone()} />
+                <Child id="sib-b" observed={props.observed.clone()} />
+            </Suspense>
+        }
+    }
+
+    let observed: Rc<RefCell<Vec<(&'static str, bool)>>> = Rc::new(RefCell::new(Vec::new()));
+    yew::Renderer::<App>::with_root_and_props(
+        gloo::utils::document().get_element_by_id("output").unwrap(),
+        AppProps {
+            observed: observed.clone(),
+        },
+    )
+    .render();
+
+    sleep(Duration::from_millis(100)).await;
+
+    let observed = observed.borrow();
+    assert_eq!(
+        observed.len(),
+        2,
+        "both sibling effects should fire exactly once: {:?}",
+        *observed
+    );
+    for (id, found) in observed.iter() {
+        assert!(
+            *found,
+            "effect for {} did not see DOM in live tree: {:?}",
+            id, *observed
+        );
+    }
+}