fix(tui/scroll): keep viewport stable during streaming

fixes #527

code-rs/tui/src/chatwidget.rs

@@ -4485,8 +4485,7 @@ impl ChatWidget<'_> {
         }
         self.request_redraw();
     }
-    /// If the user is at or near the bottom, keep following new messages.
-    /// We treat "near" as within 3 rows, matching our scroll step.
+    /// If the user is at the bottom, keep following new messages.
     fn autoscroll_if_near_bottom(&mut self) {
         layout_scroll::autoscroll_if_near_bottom(self);
     }

code-rs/tui/src/chatwidget/layout_scroll.rs

@@ -5,7 +5,7 @@ use crate::height_manager::HeightEvent;
 use ratatui::layout::Rect;
 
 pub(super) fn autoscroll_if_near_bottom(chat: &mut ChatWidget<'_>) {
-    if chat.layout.scroll_offset.get() <= 3 {
+    if chat.layout.scroll_offset.get() == 0 {
         let before = chat.layout.scroll_offset.get();
         chat.layout.scroll_offset.set(0);
         chat.bottom_pane.set_compact_compose(false);

code-rs/tui/tests/vt100_chatwidget_snapshot.rs

@@ -1206,6 +1206,61 @@ fn scroll_position_stable_when_history_grows_scrolled_up() {
     );
 }
 
+#[test]
+fn scroll_position_stable_during_streaming_when_scrolled_up() {
+    let mut harness = ChatWidgetHarness::new();
+
+    for idx in 0..6 {
+        harness.push_user_prompt(format!("User prompt #{idx}: streaming scroll"));
+        harness.push_assistant_markdown(format!(
+            "Assistant response #{idx} keeps the history tall.\n\nExtra paragraph for idx {idx}."
+        ));
+    }
+
+    let _ = render_chat_widget_to_vt100(&mut harness, 80, 24);
+    let metrics = harness_layout_metrics(&harness);
+    assert!(
+        metrics.last_max_scroll > 0,
+        "scenario must overflow the history viewport to exercise scrolling"
+    );
+
+    let offset = metrics.last_max_scroll.min(3).max(1);
+    harness_force_scroll_offset(&mut harness, offset);
+    let _ = render_chat_widget_to_vt100(&mut harness, 80, 24);
+    let before = harness_layout_metrics(&harness);
+    let before_from_top =
+        before
+            .last_max_scroll
+            .saturating_sub(before.scroll_offset.min(before.last_max_scroll));
+
+    harness.handle_event(Event {
+        id: "stream-scroll".into(),
+        event_seq: 1,
+        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
+            delta: "Streaming update line one.\nStreaming update line two.\n".into(),
+        }),
+        order: Some(OrderMeta {
+            request_ordinal: 1,
+            output_index: Some(0),
+            sequence_number: Some(1),
+        }),
+    });
+    harness.drive_commit_tick();
+
+    let _ = render_chat_widget_to_vt100(&mut harness, 80, 24);
+    let after = harness_layout_metrics(&harness);
+    let after_from_top =
+        after
+            .last_max_scroll
+            .saturating_sub(after.scroll_offset.min(after.last_max_scroll));
+
+    assert_eq!(
+        before_from_top,
+        after_from_top,
+        "streaming output should not move the viewport while scrolled"
+    );
+}
+
 #[test]
 fn multiline_final_history_line_visible_at_bottom() {
     init_tracing_once();