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

@@ -456,6 +456,43 @@ impl InteractScreen {
         }
     }
 
+    fn push_routed(&mut self, node: &AstNode) {
+        for (target, text, marker) in Self::route_node(node) {
+            match target {
+                PaneTarget::Conversation => {
+                    self.conversation.current_color = Color::Cyan;
+                    self.conversation.append_text(&text);
+                    self.conversation.pending_marker = marker;
+                    self.conversation.flush_pending();
+                },
+                PaneTarget::ConversationAssistant => {
+                    self.conversation.current_color = Color::Reset;
+                    self.conversation.append_text(&text);
+                    self.conversation.pending_marker = marker;
+                    self.conversation.flush_pending();
+                },
+                PaneTarget::Tools =>
+                    self.tools.push_line(text, Color::Yellow),
+                PaneTarget::ToolResult => {
+                    for line in text.lines().take(20) {
+                        self.tools.push_line(format!("  {}", line), Color::DarkGray);
+                    }
+                }
+            }
+        }
+    }
+
+    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();
@@ -476,38 +513,37 @@ impl InteractScreen {
             (generation, ctx.conversation().to_vec())
         };
 
-        if generation != self.last_generation || entries.len() < self.last_entries.len() {
+        // 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();
         }
 
-        let start = self.last_entries.len();
+        // Detect changed entries (streaming updates mutate the last entry)
-        for node in entries.iter().skip(start) {
+        // Walk backwards from the end, pop any that differ
-            for (target, text, marker) in Self::route_node(node) {
+        let mut pop_from = self.last_entries.len();
-                match target {
+        for i in (0..self.last_entries.len()).rev() {
-                    PaneTarget::Conversation => {
+            if i >= entries.len() {
-                        self.conversation.current_color = Color::Cyan;
+                pop_from = i;
-                        self.conversation.append_text(&text);
+                continue;
-                        self.conversation.pending_marker = marker;
-                        self.conversation.flush_pending();
-                    },
-                    PaneTarget::ConversationAssistant => {
-                        self.conversation.current_color = Color::Reset;
-                        self.conversation.append_text(&text);
-                        self.conversation.pending_marker = marker;
-                        self.conversation.flush_pending();
-                    },
-                    PaneTarget::Tools =>
-                        self.tools.push_line(text, Color::Yellow),
-                    PaneTarget::ToolResult => {
-                        for line in text.lines().take(20) {
-                            self.tools.push_line(format!("  {}", line), Color::DarkGray);
-                        }
-                    }
-                }
             }
+            // 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());
         }