feat(tui2): make transcript scrollbar draggable

Support click/drag on the transcript scrollbar track/thumb.
Use tui-scrollbar's hit testing and drag interaction state, plus a
small pointer-capture flag so dragging doesn't fall through into
transcript selection when the view becomes pinned to bottom.

Skip scrollbar hit-testing when the scrollbar is auto-hidden at bottom,
so the reserved column is not an invisible interactive region.

Move the interaction glue out of app.rs into transcript_scrollbar_ui.
Return an explicit TranscriptScrollbarMouseHandling enum, and expand
docs for reserved column layout and track/thumb theme heuristics.

Author: joshka-oai

codex-rs/tui2/src/app.rs

@@ -21,6 +21,9 @@ use crate::transcript_copy_ui::TranscriptCopyUi;
 use crate::transcript_multi_click::TranscriptMultiClick;
 use crate::transcript_scrollbar::render_transcript_scrollbar_if_active;
 use crate::transcript_scrollbar::split_transcript_area;
+use crate::transcript_scrollbar_ui::TranscriptScrollbarMouseEvent;
+use crate::transcript_scrollbar_ui::TranscriptScrollbarMouseHandling;
+use crate::transcript_scrollbar_ui::TranscriptScrollbarUi;
 use crate::transcript_selection::TRANSCRIPT_GUTTER_COLS;
 use crate::transcript_selection::TranscriptSelection;
 use crate::transcript_selection::TranscriptSelectionPoint;
@@ -337,6 +340,7 @@ pub(crate) struct App {
     transcript_view_top: usize,
     transcript_total_lines: usize,
     transcript_copy_ui: TranscriptCopyUi,
+    transcript_scrollbar_ui: TranscriptScrollbarUi,
 
     // Pager overlay state (Transcript or Static like Diff)
     pub(crate) overlay: Option<Overlay>,
@@ -502,6 +506,7 @@ impl App {
             transcript_view_top: 0,
             transcript_total_lines: 0,
             transcript_copy_ui: TranscriptCopyUi::new_with_shortcut(copy_selection_shortcut),
+            transcript_scrollbar_ui: TranscriptScrollbarUi::default(),
             overlay: None,
             deferred_history_lines: Vec::new(),
             has_emitted_history_lines: false,
@@ -866,17 +871,38 @@ impl App {
             width,
             height: transcript_height,
         };
-        let (transcript_area, _) = split_transcript_area(transcript_full_area);
+        let (transcript_area, transcript_scrollbar_area) =
+            split_transcript_area(transcript_full_area);
         let base_x = transcript_area.x.saturating_add(TRANSCRIPT_GUTTER_COLS);
         let max_x = transcript_area.right().saturating_sub(1);
 
+        if matches!(
+            self.transcript_scrollbar_ui
+                .handle_mouse_event(TranscriptScrollbarMouseEvent {
+                    tui,
+                    mouse_event,
+                    transcript_area,
+                    scrollbar_area: transcript_scrollbar_area,
+                    transcript_cells: &self.transcript_cells,
+                    transcript_view_cache: &mut self.transcript_view_cache,
+                    transcript_scroll: &mut self.transcript_scroll,
+                    transcript_view_top: &mut self.transcript_view_top,
+                    transcript_total_lines: &mut self.transcript_total_lines,
+                    mouse_scroll_state: &mut self.scroll_state,
+                }),
+            TranscriptScrollbarMouseHandling::Handled
+        ) {
+            return;
+        }
+
         // Treat the transcript as the only interactive region for transcript selection.
         //
         // This prevents clicks in the composer/footer from starting or extending a transcript
         // selection, while still allowing a left-click outside the transcript to clear an
         // existing highlight.
-        if mouse_event.row < transcript_full_area.y
-            || mouse_event.row >= transcript_full_area.bottom()
+        if !self.transcript_scrollbar_ui.pointer_capture_active()
+            && (mouse_event.row < transcript_full_area.y
+                || mouse_event.row >= transcript_full_area.bottom())
         {
             if matches!(
                 mouse_event.kind,
@@ -2124,6 +2150,7 @@ mod tests {
             transcript_copy_ui: TranscriptCopyUi::new_with_shortcut(
                 CopySelectionShortcut::CtrlShiftC,
             ),
+            transcript_scrollbar_ui: TranscriptScrollbarUi::default(),
             overlay: None,
             deferred_history_lines: Vec::new(),
             has_emitted_history_lines: false,
@@ -2175,6 +2202,7 @@ mod tests {
                 transcript_copy_ui: TranscriptCopyUi::new_with_shortcut(
                     CopySelectionShortcut::CtrlShiftC,
                 ),
+                transcript_scrollbar_ui: TranscriptScrollbarUi::default(),
                 overlay: None,
                 deferred_history_lines: Vec::new(),
                 has_emitted_history_lines: false,

codex-rs/tui2/src/lib.rs

@@ -81,6 +81,7 @@ mod transcript_copy_ui;
 mod transcript_multi_click;
 mod transcript_render;
 mod transcript_scrollbar;
+mod transcript_scrollbar_ui;
 mod transcript_selection;
 mod transcript_view_cache;
 mod tui;