Fix ACP connection and thread leak (#35670)

When you switched away from an ACP thread, the `AcpThreadView` entity
(and thus thread, and subprocess) was leaked. This happened because we
were using `cx.processor` for the `list` state callback, which uses a
strong reference.

This PR changes the callback so that it holds a weak reference, and adds
some tests and assertions at various levels to make sure we don't
reintroduce the leak in the future.

Release Notes:

- N/A

crates/agent_ui/src/acp/thread_view.rs

@@ -169,12 +169,13 @@ impl AcpThreadView {
 
         let mention_set = mention_set.clone();
 
-        let list_state = ListState::new(
-            0,
-            gpui::ListAlignment::Bottom,
-            px(2048.0),
-            cx.processor({
-                move |this: &mut Self, index: usize, window, cx| {
+        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()))
@@ -182,9 +183,9 @@ impl AcpThreadView {
                         return Empty.into_any();
                     };
                     this.render_entry(index, len, entry, window, cx)
-                }
-            }),
-        );
+                })
+            }
+        });
 
         Self {
             agent: agent.clone(),
@@ -2719,6 +2720,16 @@ mod tests {
 
     use super::*;
 
+    #[gpui::test]
+    async fn test_drop(cx: &mut TestAppContext) {
+        init_test(cx);
+
+        let (thread_view, _cx) = setup_thread_view(StubAgentServer::default(), cx).await;
+        let weak_view = thread_view.downgrade();
+        drop(thread_view);
+        assert!(!weak_view.is_upgradable());
+    }
+
     #[gpui::test]
     async fn test_notification_for_stop_event(cx: &mut TestAppContext) {
         init_test(cx);

crates/agent_ui/src/agent_panel.rs

@@ -970,13 +970,7 @@ impl AgentPanel {
                     )
                 });
 
-                this.set_active_view(
-                    ActiveView::ExternalAgentThread {
-                        thread_view: thread_view.clone(),
-                    },
-                    window,
-                    cx,
-                );
+                this.set_active_view(ActiveView::ExternalAgentThread { thread_view }, window, cx);
             })
         })
         .detach_and_log_err(cx);
@@ -1477,6 +1471,7 @@ 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, .. } => {
@@ -1488,6 +1483,9 @@ impl AgentPanel {
                     });
                 }
             }
+            ActiveView::ExternalAgentThread { thread_view } => {
+                old_acp_thread.replace(thread_view.downgrade());
+            }
             _ => {}
         }
 
@@ -1518,6 +1516,11 @@ 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);