fix(tui): make transcript scrollbar inert

## Summary
- remove app-owned transcript scrollbar click/drag state so the right gutter cannot capture mouse drags
- keep wheel scrolling, jump-to-latest, and normal transcript text selection intact
- align the behavior with opencode's session surface: scroll via wheel/key paths, no app-level scrollbar drag affordance

## Test plan
- cargo test -p deepseek-tui transcript_scrollbar_gutter_is_not_draggable --locked
- cargo test -p deepseek-tui left_down_inside_transcript_starts_selection --locked
- cargo fmt --all -- --check
- git diff --check
- GitHub CI: lint, version drift, ubuntu/macos/windows tests, npm wrapper smoke, GitGuardian

crates/tui/src/tui/app.rs

@@ -574,7 +574,6 @@ pub struct ViewportState {
     pub mouse_scroll: MouseScrollState,
     pub transcript_cache: TranscriptViewCache,
     pub transcript_selection: TranscriptSelection,
-    pub transcript_scrollbar_dragging: bool,
     pub last_transcript_area: Option<Rect>,
     pub last_transcript_top: usize,
     pub last_transcript_visible: usize,
@@ -591,7 +590,6 @@ impl Default for ViewportState {
             mouse_scroll: MouseScrollState::new(),
             transcript_cache: TranscriptViewCache::new(),
             transcript_selection: TranscriptSelection::default(),
-            transcript_scrollbar_dragging: false,
             last_transcript_area: None,
             last_transcript_top: 0,
             last_transcript_visible: 0,
@@ -2457,7 +2455,6 @@ impl App {
 
         self.viewport.pending_scroll_delta = 0;
         self.viewport.transcript_selection.clear();
-        self.viewport.transcript_scrollbar_dragging = false;
 
         self.viewport.last_transcript_area = None;
         self.viewport.last_transcript_top = 0;

crates/tui/src/tui/ui.rs

@@ -7483,20 +7483,11 @@ fn handle_mouse_event(app: &mut App, mouse: MouseEvent) -> Vec<ViewEvent> {
             }
         }
         MouseEventKind::Down(MouseButton::Left) => {
-            app.viewport.transcript_scrollbar_dragging = false;
-
             if mouse_hits_rect(mouse, app.viewport.jump_to_latest_button_area) {
                 app.scroll_to_bottom();
                 return Vec::new();
             }
 
-            if mouse_hits_transcript_scrollbar(app, mouse) {
-                app.viewport.transcript_scrollbar_dragging = true;
-                app.viewport.transcript_selection.clear();
-                scroll_transcript_to_mouse_row(app, mouse.row);
-                return Vec::new();
-            }
-
             if let Some(point) = selection_point_from_mouse(app, mouse) {
                 app.viewport.transcript_selection.anchor = Some(point);
                 app.viewport.transcript_selection.head = Some(point);
@@ -7516,21 +7507,12 @@ fn handle_mouse_event(app: &mut App, mouse: MouseEvent) -> Vec<ViewEvent> {
             }
         }
         MouseEventKind::Drag(MouseButton::Left) => {
-            if app.viewport.transcript_scrollbar_dragging {
-                scroll_transcript_to_mouse_row(app, mouse.row);
-                return Vec::new();
-            }
-
             if app.viewport.transcript_selection.dragging
                 && let Some(point) = selection_point_from_mouse(app, mouse)
             {
                 app.viewport.transcript_selection.head = Some(point);
             }
         }
-        MouseEventKind::Up(MouseButton::Left) if app.viewport.transcript_scrollbar_dragging => {
-            app.viewport.transcript_scrollbar_dragging = false;
-            app.needs_redraw = true;
-        }
         MouseEventKind::Up(MouseButton::Left) if app.viewport.transcript_selection.dragging => {
             app.viewport.transcript_selection.dragging = false;
             if selection_has_content(app) {
@@ -7546,57 +7528,6 @@ fn handle_mouse_event(app: &mut App, mouse: MouseEvent) -> Vec<ViewEvent> {
     Vec::new()
 }
 
-fn mouse_hits_transcript_scrollbar(app: &App, mouse: MouseEvent) -> bool {
-    let Some(area) = app.viewport.last_transcript_area else {
-        return false;
-    };
-    if area.width <= 1 || app.viewport.last_transcript_total <= app.viewport.last_transcript_visible
-    {
-        return false;
-    }
-
-    let scrollbar_col = area.x.saturating_add(area.width.saturating_sub(1));
-    mouse.column == scrollbar_col
-        && mouse.row >= area.y
-        && mouse.row < area.y.saturating_add(area.height)
-}
-
-fn scroll_transcript_to_mouse_row(app: &mut App, row: u16) -> bool {
-    let Some(area) = app.viewport.last_transcript_area else {
-        return false;
-    };
-    let total = app.viewport.last_transcript_total;
-    let visible = app.viewport.last_transcript_visible;
-    if area.height == 0 || total <= visible {
-        return false;
-    }
-
-    let max_start = total.saturating_sub(visible);
-    if max_start == 0 {
-        app.scroll_to_bottom();
-        return true;
-    }
-
-    let max_row = usize::from(area.height.saturating_sub(1));
-    let relative_row = usize::from(row.saturating_sub(area.y)).min(max_row);
-    let numerator = relative_row
-        .saturating_mul(max_start)
-        .saturating_add(max_row / 2);
-    // Round to the nearest transcript offset so short thumbs still feel
-    // responsive on compact terminals.
-    let top = numerator.checked_div(max_row).unwrap_or(0);
-
-    app.viewport.transcript_scroll = if top >= max_start {
-        TranscriptScroll::to_bottom()
-    } else {
-        TranscriptScroll::at_line(top)
-    };
-    app.viewport.pending_scroll_delta = 0;
-    app.user_scrolled_during_stream = !app.viewport.transcript_scroll.is_at_tail();
-    app.needs_redraw = true;
-    true
-}
-
 fn mouse_hits_rect(mouse: MouseEvent, area: Option<Rect>) -> bool {
     let Some(area) = area else {
         return false;

crates/tui/src/tui/ui/tests.rs

@@ -338,8 +338,19 @@ fn jump_to_latest_button_click_scrolls_to_tail() {
 }
 
 #[test]
-fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
+fn transcript_scrollbar_gutter_is_not_draggable() {
     let mut app = create_test_app();
+    app.history = vec![HistoryCell::Assistant {
+        content: "alpha beta".to_string(),
+        streaming: false,
+    }];
+    app.resync_history_revisions();
+    app.viewport.transcript_cache.ensure(
+        &app.history,
+        &app.history_revisions,
+        80,
+        app.transcript_render_options(),
+    );
     app.viewport.last_transcript_area = Some(Rect {
         x: 2,
         y: 5,
@@ -349,6 +360,7 @@ fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
     app.viewport.last_transcript_visible = 10;
     app.viewport.last_transcript_total = 110;
     app.viewport.transcript_scroll = TranscriptScroll::to_bottom();
+    app.user_scrolled_during_stream = false;
 
     let events = handle_mouse_event(
         &mut app,
@@ -361,12 +373,9 @@ fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
     );
 
     assert!(events.is_empty());
-    assert!(app.viewport.transcript_scrollbar_dragging);
-    assert!(!app.viewport.transcript_selection.dragging);
-    assert!(!app.viewport.transcript_scroll.is_at_tail());
-    let (_, top) = app.viewport.transcript_scroll.resolve_top(&[], 100);
-    assert_eq!(top, 0);
-    assert!(app.user_scrolled_during_stream);
+    assert!(app.viewport.transcript_selection.dragging);
+    assert!(app.viewport.transcript_scroll.is_at_tail());
+    assert!(!app.user_scrolled_during_stream);
 
     handle_mouse_event(
         &mut app,
@@ -380,6 +389,7 @@ fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
 
     assert!(app.viewport.transcript_scroll.is_at_tail());
     assert!(!app.user_scrolled_during_stream);
+    assert!(app.viewport.transcript_selection.dragging);
 
     handle_mouse_event(
         &mut app,
@@ -391,11 +401,11 @@ fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
         },
     );
 
-    assert!(!app.viewport.transcript_scrollbar_dragging);
+    assert!(!app.viewport.transcript_selection.dragging);
 }
 
 #[test]
-fn new_left_down_clears_stale_transcript_scrollbar_drag() {
+fn left_down_inside_transcript_starts_selection() {
     let mut app = create_test_app();
     app.history = vec![HistoryCell::Assistant {
         content: "alpha beta".to_string(),
@@ -410,7 +420,6 @@ fn new_left_down_clears_stale_transcript_scrollbar_drag() {
     });
     app.viewport.last_transcript_visible = 10;
     app.viewport.last_transcript_total = 110;
-    app.viewport.transcript_scrollbar_dragging = true;
 
     let events = handle_mouse_event(
         &mut app,
@@ -423,7 +432,7 @@ fn new_left_down_clears_stale_transcript_scrollbar_drag() {
     );
 
     assert!(events.is_empty());
-    assert!(!app.viewport.transcript_scrollbar_dragging);
+    assert!(app.viewport.transcript_selection.dragging);
 }
 
 #[test]