Fix chat display: restore incremental sync with change detection

sync_from_agent now detects changed entries by comparing token counts
(cheap proxy for content changes during streaming). Changed entries
get popped and re-pushed. Extracted push_routed/pop_routed helpers.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/chat.rs

@@ -456,35 +456,7 @@ impl InteractScreen {
         }
     }
 
-    fn sync_from_agent(&mut self) {
+    fn push_routed(&mut self, node: &AstNode) {
-        for _ in 0..self.pending_display_count {
-            self.conversation.pop_line();
-        }
-        self.pending_display_count = 0;
-
-        let (generation, entries) = {
-            let st = match self.agent.state.try_lock() {
-                Ok(st) => st,
-                Err(_) => return,
-            };
-            let generation = st.generation;
-            drop(st);
-            let ctx = match self.agent.context.try_lock() {
-                Ok(ctx) => ctx,
-                Err(_) => return,
-            };
-            (generation, ctx.conversation().to_vec())
-        };
-
-        if generation != self.last_generation || entries.len() < self.last_entries.len() {
-            self.conversation = PaneState::new(true);
-            self.autonomous = PaneState::new(true);
-            self.tools = PaneState::new(false);
-            self.last_entries.clear();
-        }
-
-        let start = self.last_entries.len();
-        for node in entries.iter().skip(start) {
         for (target, text, marker) in Self::route_node(node) {
             match target {
                 PaneTarget::Conversation => {
@@ -508,6 +480,70 @@ impl InteractScreen {
                 }
             }
         }
+    }
+
+    fn pop_routed(&mut self, node: &AstNode) {
+        for (target, _, _) in Self::route_node(node) {
+            match target {
+                PaneTarget::Conversation | PaneTarget::ConversationAssistant
+                    => self.conversation.pop_line(),
+                PaneTarget::Tools | PaneTarget::ToolResult
+                    => self.tools.pop_line(),
+            }
+        }
+    }
+
+    fn sync_from_agent(&mut self) {
+        for _ in 0..self.pending_display_count {
+            self.conversation.pop_line();
+        }
+        self.pending_display_count = 0;
+
+        let (generation, entries) = {
+            let st = match self.agent.state.try_lock() {
+                Ok(st) => st,
+                Err(_) => return,
+            };
+            let generation = st.generation;
+            drop(st);
+            let ctx = match self.agent.context.try_lock() {
+                Ok(ctx) => ctx,
+                Err(_) => return,
+            };
+            (generation, ctx.conversation().to_vec())
+        };
+
+        // Full reset on generation change
+        if generation != self.last_generation {
+            self.conversation = PaneState::new(true);
+            self.autonomous = PaneState::new(true);
+            self.tools = PaneState::new(false);
+            self.last_entries.clear();
+        }
+
+        // Detect changed entries (streaming updates mutate the last entry)
+        // Walk backwards from the end, pop any that differ
+        let mut pop_from = self.last_entries.len();
+        for i in (0..self.last_entries.len()).rev() {
+            if i >= entries.len() {
+                pop_from = i;
+                continue;
+            }
+            // Compare token count as a cheap change detector
+            if self.last_entries[i].tokens() != entries[i].tokens() {
+                pop_from = i;
+            } else {
+                break; // entries before this haven't changed
+            }
+        }
+        while self.last_entries.len() > pop_from {
+            let popped = self.last_entries.pop().unwrap();
+            self.pop_routed(&popped);
+        }
+
+        // Push new/changed entries
+        for node in entries.iter().skip(self.last_entries.len()) {
+            self.push_routed(node);
             self.last_entries.push(node.clone());
         }