Simplify ListState API (#35685)

Follow up to: https://github.com/zed-industries/zed/pull/35670,
simplifies the List state APIs so you no longer have to worry about
strong vs. weak pointers when rendering list items.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent_ui/src/acp/thread_view.rs

@@ -173,23 +173,7 @@ impl AcpThreadView {
 
         let mention_set = mention_set.clone();
 
-        let list_state = ListState::new(0, gpui::ListAlignment::Bottom, px(2048.0), {
-            let this = cx.entity().downgrade();
-            move |index: usize, window, cx| {
-                let Some(this) = this.upgrade() else {
-                    return Empty.into_any();
-                };
-                this.update(cx, |this, cx| {
-                    let Some((entry, len)) = this.thread().and_then(|thread| {
-                        let entries = &thread.read(cx).entries();
-                        Some((entries.get(index)?, entries.len()))
-                    }) else {
-                        return Empty.into_any();
-                    };
-                    this.render_entry(index, len, entry, window, cx)
-                })
-            }
-        });
+        let list_state = ListState::new(0, gpui::ListAlignment::Bottom, px(2048.0));
 
         Self {
             agent: agent.clone(),
@@ -2552,10 +2536,21 @@ impl Render for AcpThreadView {
                     v_flex().flex_1().map(|this| {
                         if self.list_state.item_count() > 0 {
                             this.child(
-                                list(self.list_state.clone())
-                                    .with_sizing_behavior(gpui::ListSizingBehavior::Auto)
-                                    .flex_grow()
-                                    .into_any(),
+                                list(
+                                    self.list_state.clone(),
+                                    cx.processor(|this, index: usize, window, cx| {
+                                        let Some((entry, len)) = this.thread().and_then(|thread| {
+                                            let entries = &thread.read(cx).entries();
+                                            Some((entries.get(index)?, entries.len()))
+                                        }) else {
+                                            return Empty.into_any();
+                                        };
+                                        this.render_entry(index, len, entry, window, cx)
+                                    }),
+                                )
+                                .with_sizing_behavior(gpui::ListSizingBehavior::Auto)
+                                .flex_grow()
+                                .into_any(),
                             )
                             .children(match thread_clone.read(cx).status() {
                                 ThreadStatus::Idle | ThreadStatus::WaitingForToolConfirmation => {

crates/agent_ui/src/active_thread.rs

@@ -780,13 +780,7 @@ impl ActiveThread {
             cx.observe_global::<SettingsStore>(|_, cx| cx.notify()),
         ];
 
-        let list_state = ListState::new(0, ListAlignment::Bottom, px(2048.), {
-            let this = cx.entity().downgrade();
-            move |ix, window: &mut Window, cx: &mut App| {
-                this.update(cx, |this, cx| this.render_message(ix, window, cx))
-                    .unwrap()
-            }
-        });
+        let list_state = ListState::new(0, ListAlignment::Bottom, px(2048.));
 
         let workspace_subscription = if let Some(workspace) = workspace.upgrade() {
             Some(cx.observe_release(&workspace, |this, _, cx| {
@@ -1846,7 +1840,12 @@ impl ActiveThread {
             )))
     }
 
-    fn render_message(&self, ix: usize, window: &mut Window, cx: &mut Context<Self>) -> AnyElement {
+    fn render_message(
+        &mut self,
+        ix: usize,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) -> AnyElement {
         let message_id = self.messages[ix];
         let workspace = self.workspace.clone();
         let thread = self.thread.read(cx);
@@ -3613,7 +3612,7 @@ impl Render for ActiveThread {
                     this.hide_scrollbar_later(cx);
                 }),
             )
-            .child(list(self.list_state.clone()).flex_grow())
+            .child(list(self.list_state.clone(), cx.processor(Self::render_message)).flex_grow())
             .when_some(self.render_vertical_scrollbar(cx), |this, scrollbar| {
                 this.child(scrollbar)
             })

crates/agent_ui/src/agent_panel.rs

@@ -1471,7 +1471,6 @@ impl AgentPanel {
 
         let current_is_special = current_is_history || current_is_config;
         let new_is_special = new_is_history || new_is_config;
-        let mut old_acp_thread = None;
 
         match &self.active_view {
             ActiveView::Thread { thread, .. } => {
@@ -1483,9 +1482,6 @@ impl AgentPanel {
                     });
                 }
             }
-            ActiveView::ExternalAgentThread { thread_view } => {
-                old_acp_thread.replace(thread_view.downgrade());
-            }
             _ => {}
         }
 
@@ -1516,11 +1512,6 @@ impl AgentPanel {
             self.active_view = new_view;
         }
 
-        debug_assert!(
-            old_acp_thread.map_or(true, |thread| !thread.is_upgradable()),
-            "AcpThreadView leaked"
-        );
-
         self.acp_message_history.borrow_mut().reset_position();
 
         self.focus_handle(cx).focus(window);