feat(tui): support transcript scrollbar dragging

crates/tui/src/tui/app.rs

@@ -575,6 +575,7 @@ 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,
@@ -591,6 +592,7 @@ 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,

crates/tui/src/tui/ui.rs

@@ -7324,6 +7324,13 @@ fn handle_mouse_event(app: &mut App, mouse: MouseEvent) -> Vec<ViewEvent> {
                 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);
@@ -7343,12 +7350,21 @@ 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) {
@@ -7364,6 +7380,61 @@ 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 top = if max_row == 0 {
+        0
+    } else {
+        // Round to the nearest transcript offset so short thumbs still feel
+        // responsive on compact terminals.
+        (relative_row
+            .saturating_mul(max_start)
+            .saturating_add(max_row / 2))
+            / max_row
+    };
+
+    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

@@ -302,6 +302,63 @@ fn jump_to_latest_button_click_scrolls_to_tail() {
     assert!(!app.viewport.transcript_selection.dragging);
 }
 
+#[test]
+fn transcript_scrollbar_drag_maps_mouse_row_to_scroll_position() {
+    let mut app = create_test_app();
+    app.viewport.last_transcript_area = Some(Rect {
+        x: 2,
+        y: 5,
+        width: 20,
+        height: 10,
+    });
+    app.viewport.last_transcript_visible = 10;
+    app.viewport.last_transcript_total = 110;
+    app.viewport.transcript_scroll = TranscriptScroll::to_bottom();
+
+    let events = handle_mouse_event(
+        &mut app,
+        MouseEvent {
+            kind: MouseEventKind::Down(MouseButton::Left),
+            column: 21,
+            row: 5,
+            modifiers: KeyModifiers::NONE,
+        },
+    );
+
+    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);
+
+    handle_mouse_event(
+        &mut app,
+        MouseEvent {
+            kind: MouseEventKind::Drag(MouseButton::Left),
+            column: 21,
+            row: 14,
+            modifiers: KeyModifiers::NONE,
+        },
+    );
+
+    assert!(app.viewport.transcript_scroll.is_at_tail());
+    assert!(!app.user_scrolled_during_stream);
+
+    handle_mouse_event(
+        &mut app,
+        MouseEvent {
+            kind: MouseEventKind::Up(MouseButton::Left),
+            column: 21,
+            row: 14,
+            modifiers: KeyModifiers::NONE,
+        },
+    );
+
+    assert!(!app.viewport.transcript_scrollbar_dragging);
+}
+
 #[test]
 fn right_click_opens_context_menu() {
     let mut app = create_test_app();