fix: do not show Transition fallback on 2nd change if 1st change resolved synchronously (closes #4492, closes #3868) (#4495)

examples/regression/Cargo.toml

@@ -17,7 +17,7 @@ leptos_router = { path = "../../router" }
 serde = { version = "1.0", features = ["derive"] }
 thiserror = "2.0.12"
 tokio = { version = "1.39", features = [ "rt-multi-thread", "macros", "time" ], optional = true }
-wasm-bindgen = "0.2.92"
+wasm-bindgen = "0.2.106"
 
 [features]
 hydrate = [

examples/regression/e2e/features/issue_4492.feature

@@ -0,0 +1,38 @@
+@check_issue_4492
+Feature: Regression test for issue #4492
+
+	Scenario: Scenario A should show Loading once on first load.
+		Given I see the app
+		And I can access regression test 4492
+		When I click the button a-toggle
+		Then I see a-result has the text Loading...
+		When I wait 100ms
+		Then I see a-result has the text 0
+		When I click the button a-button
+		Then I see a-result has the text 0
+		When I wait 100ms
+		Then I see a-result has the text 1
+
+	Scenario: Scenario B should never show Loading
+		Given I see the app
+		And I can access regression test 4492
+		When I click the button b-toggle
+		Then I see b-result has the text 0
+		When I click the button b-button
+		Then I see b-result has the text 0
+		When I wait 100ms
+		Then I see b-result has the text 1
+		When I click the button b-button
+		Then I see b-result has the text 1
+		When I wait 100ms
+		Then I see b-result has the text 2
+
+	Scenario: Scenario C should never show Loading
+		Given I see the app
+		And I can access regression test 4492
+		When I click the button c-toggle
+		Then I see c-result has the text 0
+		When I click the button c-button
+		Then I see c-result has the text 42
+		When I wait 100ms
+		Then I see c-result has the text 1

examples/regression/e2e/tests/fixtures/action.rs

@@ -15,3 +15,9 @@ pub async fn click_link(client: &Client, text: &str) -> Result<()> {
     link.click().await?;
     Ok(())
 }
+
+pub async fn click_button(client: &Client, id: &str) -> Result<()> {
+    let btn = find::element_by_id(&client, &id).await?;
+    btn.click().await?;
+    Ok(())
+}

examples/regression/e2e/tests/fixtures/check.rs

@@ -7,7 +7,15 @@ pub async fn result_text_is(
     client: &Client,
     expected_text: &str,
 ) -> Result<()> {
-    let actual = find::text_at_id(client, "result").await?;
+    element_text_is(client, "result", expected_text).await
+}
+
+pub async fn element_text_is(
+    client: &Client,
+    id: &str,
+    expected_text: &str,
+) -> Result<()> {
+    let actual = find::text_at_id(client, id).await?;
     assert_eq!(&actual, expected_text);
     Ok(())
 }

examples/regression/e2e/tests/fixtures/world/action_steps.rs

@@ -20,6 +20,14 @@ async fn i_select_the_link(world: &mut AppWorld, text: String) -> Result<()> {
     Ok(())
 }
 
+#[when(regex = "^I click the button (.*)$")]
+async fn i_click_the_button(world: &mut AppWorld, id: String) -> Result<()> {
+    let client = &world.client;
+    action::click_button(client, &id).await?;
+
+    Ok(())
+}
+
 #[given(expr = "I select the following links")]
 #[when(expr = "I select the following links")]
 async fn i_select_the_following_links(
@@ -54,3 +62,10 @@ async fn i_go_back(world: &mut AppWorld) -> Result<()> {
 
     Ok(())
 }
+
+#[when(regex = r"^I wait (\d+)ms$")]
+async fn i_wait_ms(_world: &mut AppWorld, ms: u64) -> Result<()> {
+    tokio::time::sleep(std::time::Duration::from_millis(ms)).await;
+
+    Ok(())
+}

examples/regression/e2e/tests/fixtures/world/check_steps.rs

@@ -19,6 +19,17 @@ async fn i_see_the_result_is_the_string(
     Ok(())
 }
 
+#[then(regex = r"^I see ([\w-]+) has the text (.*)$")]
+async fn i_see_element_has_text(
+    world: &mut AppWorld,
+    id: String,
+    text: String,
+) -> Result<()> {
+    let client = &world.client;
+    check::element_text_is(client, &id, &text).await?;
+    Ok(())
+}
+
 #[then(regex = r"^I see the navbar$")]
 async fn i_see_the_navbar(world: &mut AppWorld) -> Result<()> {
     let client = &world.client;

examples/regression/src/app.rs

@@ -1,7 +1,7 @@
 use crate::{
     issue_4005::Routes4005, issue_4088::Routes4088, issue_4217::Routes4217,
     issue_4285::Routes4285, issue_4296::Routes4296, issue_4324::Routes4324,
-    pr_4015::Routes4015, pr_4091::Routes4091,
+    issue_4492::Routes4492, pr_4015::Routes4015, pr_4091::Routes4091,
 };
 use leptos::prelude::*;
 use leptos_meta::{MetaTags, *};
@@ -48,6 +48,7 @@ pub fn App() -> impl IntoView {
                     <Routes4285/>
                     <Routes4296/>
                     <Routes4324/>
+                    <Routes4492/>
                 </Routes>
             </main>
         </Router>
@@ -75,6 +76,7 @@ fn HomePage() -> impl IntoView {
                 <li><a href="/4285/">"4285"</a></li>
                 <li><a href="/4296/">"4296"</a></li>
                 <li><a href="/4324/">"4324"</a></li>
+                <li><a href="/4492/">"4492"</a></li>
             </ul>
         </nav>
     }

examples/regression/src/issue_4492.rs

@@ -0,0 +1,114 @@
+use leptos::prelude::*;
+#[allow(unused_imports)]
+use leptos_router::{
+    components::Route, path, MatchNestedRoutes, NavigateOptions,
+};
+
+#[component]
+pub fn Routes4492() -> impl MatchNestedRoutes + Clone {
+    view! {
+        <Route path=path!("4492") view=Issue4492/>
+    }
+    .into_inner()
+}
+
+#[component]
+fn Issue4492() -> impl IntoView {
+    let show_a = RwSignal::new(false);
+    let show_b = RwSignal::new(false);
+    let show_c = RwSignal::new(false);
+
+    view! {
+        <button id="a-toggle" on:click=move |_| show_a.set(!show_a.get())>"Toggle A"</button>
+        <button id="b-toggle" on:click=move |_| show_b.set(!show_b.get())>"Toggle B"</button>
+        <button id="c-toggle" on:click=move |_| show_c.set(!show_c.get())>"Toggle C"</button>
+
+        <Show when=move || show_a.get()>
+            <ScenarioA/>
+        </Show>
+        <Show when=move || show_b.get()>
+            <ScenarioB/>
+        </Show>
+        <Show when=move || show_c.get()>
+            <ScenarioC/>
+        </Show>
+    }
+}
+
+#[component]
+fn ScenarioA() -> impl IntoView {
+    // scenario A: one truly-async resource is read on click
+    let counter = RwSignal::new(0);
+    let resource = Resource::new(
+        move || counter.get(),
+        |count| async move {
+            sleep(50).await.unwrap();
+            count
+        },
+    );
+    view! {
+        <Transition fallback=|| view! { <p id="a-result">"Loading..."</p> }>
+            <p id="a-result">{resource}</p>
+        </Transition>
+        <button id="a-button" on:click=move |_| *counter.write() += 1>"+1"</button>
+    }
+}
+
+#[component]
+fn ScenarioB() -> impl IntoView {
+    // scenario B: resource immediately available first time, then after 250ms
+    let counter = RwSignal::new(0);
+    let resource = Resource::new(
+        move || counter.get(),
+        |count| async move {
+            if count == 0 {
+                count
+            } else {
+                sleep(50).await.unwrap();
+                count
+            }
+        },
+    );
+    view! {
+        <Transition fallback=|| view! { <p id="b-result">"Loading..."</p> }>
+            <p id="b-result">{resource}</p>
+        </Transition>
+        <button id="b-button" on:click=move |_| *counter.write() += 1>"+1"</button>
+    }
+}
+
+#[component]
+fn ScenarioC() -> impl IntoView {
+    // scenario C: not even a resource on the first run, just a value
+    // see https://github.com/leptos-rs/leptos/issues/3868
+    let counter = RwSignal::new(0);
+    let s_res = StoredValue::new(None::<ArcLocalResource<i32>>);
+    let resource = move || {
+        let count = counter.get();
+        if count == 0 {
+            count
+        } else {
+            let r = s_res.get_value().unwrap_or_else(|| {
+                let res = ArcLocalResource::new(move || async move {
+                    sleep(50).await.unwrap();
+                    count
+                });
+                s_res.set_value(Some(res.clone()));
+                res
+            });
+            r.get().unwrap_or(42)
+        }
+    };
+    view! {
+        <Transition fallback=|| view! { <p id="c-result">"Loading..."</p> }>
+            <p id="c-result">{resource}</p>
+        </Transition>
+        <button id="c-button" on:click=move |_| *counter.write() += 1>"+1"</button>
+    }
+}
+
+#[server]
+async fn sleep(ms: u64) -> Result<(), ServerFnError> {
+    tokio::time::sleep(std::time::Duration::from_millis(ms)).await;
+    Ok(())
+}

examples/regression/src/lib.rs

@@ -5,6 +5,7 @@ mod issue_4217;
 mod issue_4285;
 mod issue_4296;
 mod issue_4324;
+mod issue_4492;
 mod pr_4015;
 mod pr_4091;
 

leptos/src/suspense_component.rs

@@ -15,7 +15,10 @@ use reactive_graph::{
     effect::RenderEffect,
     owner::{provide_context, use_context, Owner},
     signal::ArcRwSignal,
-    traits::{Dispose, Get, Read, ReadUntracked, Track, With, WriteValue},
+    traits::{
+        Dispose, Get, Read, ReadUntracked, Track, With, WithUntracked,
+        WriteValue,
+    },
 };
 use slotmap::{DefaultKey, SlotMap};
 use std::sync::{Arc, Mutex};
@@ -119,14 +122,19 @@ where
         provide_context(SuspenseContext {
             tasks: tasks.clone(),
         });
-        let none_pending = ArcMemo::new(move |prev: Option<&bool>| {
+        let none_pending = ArcMemo::new({
+            let tasks = tasks.clone();
+            move |prev: Option<&bool>| {
                 tasks.track();
                 if prev.is_none() && starts_local {
                     false
                 } else {
                     tasks.with(SlotMap::is_empty)
                 }
+            }
         });
+        let has_tasks =
+            Arc::new(move || !tasks.with_untracked(SlotMap::is_empty));
 
         OwnedView::new(SuspenseBoundary::<false, _, _> {
             id,
@@ -134,6 +142,7 @@ where
             fallback,
             children,
             error_boundary_parent,
+            has_tasks,
         })
     })
 }
@@ -156,6 +165,7 @@ pub(crate) struct SuspenseBoundary<const TRANSITION: bool, Fal, Chil> {
     pub fallback: Fal,
     pub children: Chil,
     pub error_boundary_parent: Option<ErrorBoundarySuspendedChildren>,
+    pub has_tasks: Arc<dyn Fn() -> bool + Send + Sync>,
 }
 
 impl<const TRANSITION: bool, Fal, Chil> Render
@@ -192,12 +202,26 @@ where
                 outer_owner.clone(),
             );
 
-            if let Some(mut state) = prev {
+            let state = if let Some(mut state) = prev {
                 this.rebuild(&mut state);
                 state
             } else {
                 this.build()
+            };
+
+            if nth_run == 1 && !(self.has_tasks)() {
+                // if this is the first run, and there are no pending resources at this point,
+                // it means that there were no actually-async resources read while rendering the children
+                // this means that we're effectively on the settled second run: none_pending
+                // won't change false => true and cause this to rerender (and therefore increment nth_run)
+                //
+                // we increment it manually here so that future resource changes won't cause the transition fallback
+                // to be displayed for the first time
+                // see https://github.com/leptos-rs/leptos/issues/3868, https://github.com/leptos-rs/leptos/issues/4492
+                nth_run += 1;
             }
+
+            state
         })
     }
 
@@ -235,6 +259,7 @@ where
             fallback,
             children,
             error_boundary_parent,
+            has_tasks,
         } = self;
         SuspenseBoundary {
             id,
@@ -242,6 +267,7 @@ where
             fallback,
             children: children.add_any_attr(attr),
             error_boundary_parent,
+            has_tasks,
         }
     }
 }

leptos/src/transition.rs

@@ -10,10 +10,11 @@ use reactive_graph::{
     effect::Effect,
     owner::{provide_context, use_context, Owner},
     signal::ArcRwSignal,
-    traits::{Get, Set, Track, With},
+    traits::{Get, Set, Track, With, WithUntracked},
     wrappers::write::SignalSetter,
 };
 use slotmap::{DefaultKey, SlotMap};
+use std::sync::Arc;
 use tachys::reactive_graph::OwnedView;
 
 /// If any [`Resource`](crate::prelude::Resource) is read in the `children` of this
@@ -104,14 +105,19 @@ where
         provide_context(SuspenseContext {
             tasks: tasks.clone(),
         });
-        let none_pending = ArcMemo::new(move |prev: Option<&bool>| {
+        let none_pending = ArcMemo::new({
+            let tasks = tasks.clone();
+            move |prev: Option<&bool>| {
                 tasks.track();
                 if prev.is_none() && starts_local {
                     false
                 } else {
                     tasks.with(SlotMap::is_empty)
                 }
+            }
         });
+        let has_tasks =
+            Arc::new(move || !tasks.with_untracked(SlotMap::is_empty));
         if let Some(set_pending) = set_pending {
             Effect::new_isomorphic({
                 let none_pending = none_pending.clone();
@@ -127,6 +133,7 @@ where
             fallback,
             children,
             error_boundary_parent,
+            has_tasks,
         })
     })
 }

leptos_server/src/once_resource.rs

@@ -15,7 +15,7 @@ use codee::{
     Decoder, Encoder,
 };
 use core::{fmt::Debug, marker::PhantomData};
-use futures::Future;
+use futures::{Future, FutureExt};
 use or_poisoned::OrPoisoned;
 use reactive_graph::{
     computed::{
@@ -258,11 +258,17 @@ where
         if let Some(suspense_context) = use_context::<SuspenseContext>() {
             if self.value.read().or_poisoned().is_none() {
                 let handle = suspense_context.task_id();
-                let ready = SpecialNonReactiveFuture::new(self.ready());
+                let mut ready =
+                    Box::pin(SpecialNonReactiveFuture::new(self.ready()));
+                match ready.as_mut().now_or_never() {
+                    Some(_) => drop(handle),
+                    None => {
                         reactive_graph::spawn(async move {
                             ready.await;
                             drop(handle);
                         });
+                    }
+                }
                 self.suspenses.write().or_poisoned().push(suspense_context);
             }
         }

reactive_graph/src/computed/async_derived/arc_async_derived.rs

@@ -632,12 +632,29 @@ impl<T: 'static> ReadUntracked for ArcAsyncDerived<T> {
 
     fn try_read_untracked(&self) -> Option<Self::Value> {
         if let Some(suspense_context) = use_context::<SuspenseContext>() {
+            // create a handle to register it with suspense
             let handle = suspense_context.task_id();
-            let ready = SpecialNonReactiveFuture::new(self.ready());
+
+            // check if the task is *already* ready
+            let mut ready =
+                Box::pin(SpecialNonReactiveFuture::new(self.ready()));
+            match ready.as_mut().now_or_never() {
+                Some(_) => {
+                    // if it's already ready, drop the handle immediately
+                    // this will immediately notify the suspense context that it's complete
+                    drop(handle);
+                }
+                None => {
+                    // otherwise, spawn a task to wait for it to be ready, then drop the handle,
+                    // which will notify the suspense
                     crate::spawn(async move {
                         ready.await;
                         drop(handle);
                     });
+                }
+            }
+
+            // register the suspense context with our list of them, to be notified later if this re-runs
             self.inner
                 .write()
                 .or_poisoned()