chat: double-buffered sync with content-length diffing

Store content lengths of rendered entries. On each tick:
- Generation changed → full pane reset
- Entries removed → pop from tail
- Last entry content length changed → pop and re-render (streaming)
- New entries → route and push

PaneState gains pop_line() for removing the last rendered entry.
This handles streaming (last entry growing), compaction (generation
bump), and normal appends.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/chat.rs

@@ -78,9 +78,10 @@ pub(crate) struct InteractScreen {
     pub(crate) turn_started: Option<std::time::Instant>,
     pub(crate) call_started: Option<std::time::Instant>,
     pub(crate) call_timeout_secs: u64,
-    // State sync with agent
+    // State sync with agent — double buffer
     last_generation: u64,
-    last_entry_count: usize,
+    /// Content lengths of rendered entries — for detecting changes
+    last_entry_lengths: Vec<usize>,
     /// Reference to agent for state sync
     agent: std::sync::Arc<tokio::sync::Mutex<crate::agent::Agent>>,
 }
@@ -101,7 +102,7 @@ impl InteractScreen {
             call_started: None,
             call_timeout_secs: 60,
             last_generation: 0,
-            last_entry_count: 0,
+            last_entry_lengths: Vec::new(),
             agent,
         }
     }
@@ -110,41 +111,63 @@ impl InteractScreen {
     fn sync_from_agent(&mut self) {
         let agent = self.agent.blocking_lock();
         let gen = agent.generation;
-        let count = agent.entries().len();
+        let entries = agent.entries();
 
-        // Phase 1: detect desync — pop entries from panes to match
-        if gen != self.last_generation || count < self.last_entry_count {
-            // Compaction or mutation happened — full reset
-            // (could be smarter and pop from front, but reset is safe)
+        // Phase 1: detect desync and pop
+        if gen != self.last_generation {
             self.conversation = PaneState::new(true);
             self.autonomous = PaneState::new(true);
             self.tools = PaneState::new(false);
-            self.last_entry_count = 0;
-        }
-
-        // Phase 2: route new entries to panes
-        for entry in agent.entries().iter().skip(self.last_entry_count) {
-            match route_entry(entry) {
-                Some((PaneTarget::Conversation, text, marker)) => {
-                    self.conversation.push_line_with_marker(text, Color::Cyan, marker);
+            self.last_entry_lengths.clear();
+        } else {
+            // Pop entries from the tail that were removed or changed
+            while self.last_entry_lengths.len() > entries.len() {
+                self.last_entry_lengths.pop();
+                // TODO: pop from correct pane
+            }
+            // Check if last entry changed (streaming)
+            if let (Some(&last_len), Some(entry)) = (
+                self.last_entry_lengths.last(),
+                entries.get(self.last_entry_lengths.len() - 1),
+            ) {
+                let cur_len = entry.message().content_text().len();
+                if cur_len != last_len {
+                    // Last entry changed — pop and re-render
+                    self.last_entry_lengths.pop();
+                    self.conversation.pop_line();
                 }
-                Some((PaneTarget::ConversationAssistant, text, marker)) => {
-                    self.conversation.push_line_with_marker(text, Color::Reset, marker);
-                }
-                Some((PaneTarget::Tools, text, _)) => {
-                    self.tools.push_line(text, Color::Yellow);
-                }
-                Some((PaneTarget::ToolResult, text, _)) => {
-                    for line in text.lines().take(20) {
-                        self.tools.push_line(format!("  {}", line), Color::DarkGray);
-                    }
-                }
-                None => {} // skip (memory, system, system-reminder)
             }
         }
 
+        // Phase 2: push new/changed entries
+        let start = self.last_entry_lengths.len();
+        for entry in entries.iter().skip(start) {
+            let msg = entry.message();
+            let text_len = msg.content_text().len();
+
+            if let Some((target, text, marker)) = route_entry(entry) {
+                match target {
+                    PaneTarget::Conversation => {
+                        self.conversation.push_line_with_marker(text, Color::Cyan, marker);
+                    }
+                    PaneTarget::ConversationAssistant => {
+                        self.conversation.push_line_with_marker(text, Color::Reset, marker);
+                    }
+                    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);
+                        }
+                    }
+                }
+            }
+
+            self.last_entry_lengths.push(text_len);
+        }
+
         self.last_generation = gen;
-        self.last_entry_count = count;
     }
 
     /// Process a UiMessage — update pane state.

src/user/mod.rs

@@ -190,6 +190,11 @@ impl PaneState {
         self.evict();
     }
 
+    pub(crate) fn pop_line(&mut self) {
+        self.lines.pop();
+        self.markers.pop();
+    }
+
     fn scroll_up(&mut self, n: u16) {
         self.scroll = self.scroll.saturating_sub(n);
         self.pinned = true;