Analysis notes: UI desync pop/push line count mismatch

Documents the root cause of the streaming display bug —
pop removes 1 line per entry but push produces N lines
(markdown, tool results). Includes concrete fix approach
using per-entry line count tracking.

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

.claude/ui-desync-analysis.md

@@ -0,0 +1,100 @@
+# UI Desync Analysis — Pending Input + Entry Pop (2026-04-07)
+
+## Context
+
+The F1 conversation pane has a desync bug where entries aren't
+properly removed when they change (streaming updates, compaction).
+Qwen's fix restored the pending_display_count approach for pending
+input, which works. The remaining issue is the **entry-level pop**.
+
+## The Bug: Pop/Push Line Count Mismatch
+
+In `sync_from_agent()` (chat.rs), Phase 1 pops changed entries and
+Phase 2 pushes new ones. The push and pop paths produce different
+numbers of display lines for the same entry.
+
+### Push path (Phase 2, lines 512-536):
+
+- **Conversation/ConversationAssistant**: `append_text(&text)` +
+  `flush_pending()`. In markdown mode, `flush_pending` runs
+  `parse_markdown()` which can produce N lines from the input text
+  (paragraph breaks, code blocks, etc.)
+
+- **Tools**: `push_line(text, Color::Yellow)` — exactly 1 line.
+
+- **ToolResult**: `text.lines().take(20)` — up to 20 lines, each
+  pushed separately.
+
+### Pop path (Phase 1, lines 497-507):
+
+```rust
+for (target, _, _) in Self::route_entry(&popped) {
+    match target {
+        PaneTarget::Conversation | PaneTarget::ConversationAssistant
+            => self.conversation.pop_line(),
+        PaneTarget::Tools | PaneTarget::ToolResult
+            => self.tools.pop_line(),
+    }
+}
+```
+
+This pops **one line per route_entry item**, not per display line.
+
+### The mismatch:
+
+| Target              | Push lines | Pop lines | Delta    |
+|---------------------|-----------|-----------|----------|
+| Conversation (md)   | N (from parse_markdown) | 1 | N-1 stale lines |
+| Tools               | 1         | 1         | OK       |
+| ToolResult          | up to 20  | 1         | up to 19 stale lines |
+
+## When it matters
+
+During **streaming**: the last assistant entry is modified on each
+token batch. `sync_from_agent` detects the mismatch (line 485),
+pops the old entry (1 line), pushes the new entry (N lines from
+markdown). Next update: pops 1 line again, but there are now N
+lines from the previous push. Stale lines accumulate.
+
+## Fix approach
+
+Track the actual number of display lines each entry produced.
+Simplest: snapshot `conversation.lines.len()` before and after
+pushing each entry in Phase 2. Store the deltas in a parallel
+`Vec<(usize, usize)>` (conversation_lines, tools_lines) alongside
+`last_entries`. Use these recorded counts when popping in Phase 1.
+
+```rust
+// Phase 2: push new entries (modified)
+let conv_before = self.conversation.lines.len();
+let tools_before = self.tools.lines.len();
+for (target, text, marker) in Self::route_entry(entry) {
+    // ... existing push logic ...
+}
+let conv_delta = self.conversation.lines.len() - conv_before;
+let tools_delta = self.tools.lines.len() - tools_before;
+self.last_entry_line_counts.push((conv_delta, tools_delta));
+
+// Phase 1: pop (modified)
+while self.last_entries.len() > pop {
+    self.last_entries.pop();
+    let (conv_lines, tools_lines) = self.last_entry_line_counts.pop().unwrap();
+    for _ in 0..conv_lines { self.conversation.pop_line(); }
+    for _ in 0..tools_lines { self.tools.pop_line(); }
+}
+```
+
+## Note on PaneState::evict()
+
+`evict()` can remove old lines from the beginning when the pane
+exceeds `MAX_PANE_LINES` (10,000). This could make the delta-based
+approach slightly inaccurate for very old entries. But we only pop
+recent entries (streaming updates are always at the tail), so
+eviction doesn't affect the entries we're popping.
+
+## Files
+
+- `src/user/chat.rs:461-550` — sync_from_agent
+- `src/user/chat.rs:282-298` — PaneState::append_text (markdown path)
+- `src/user/chat.rs:261-276` — PaneState::flush_pending
+- `src/user/chat.rs:206-219` — parse_markdown