fix(tui2): render copy pill at viewport bottom (#8716)

When the selection ends on the last visible row, the copy affordance had
no space below and never rendered. Fall back to placing it above (or on
the same row for 1-row viewports) and add a regression test.

codex-rs/tui2/src/transcript_copy_ui.rs

@@ -249,8 +249,16 @@ impl TranscriptCopyUi {
         let Some((y, to_x)) = last_visible_segment else {
             return;
         };
-        // Place the pill on the row below the last visible selection segment.
-        let Some(y) = y.checked_add(1).filter(|y| *y < area.bottom()) else {
+        // Prefer placing the pill on the row below the last visible selection segment. If the
+        // selection ends on the last visible row, fall back to placing it above (or, if the view
+        // is only one row tall, on the same row).
+        let Some(y) = y
+            .checked_add(1)
+            .filter(|y| *y < area.bottom())
+            .or_else(|| y.checked_sub(1).filter(|y| *y >= area.y))
+            .or(Some(y))
+            .filter(|y| *y < area.bottom())
+        else {
             return;
         };
 
@@ -329,4 +337,26 @@ mod tests {
         assert!(!rendered.contains("ctrl + shift + c"));
         assert!(ui.affordance_rect.is_some());
     }
+
+    #[test]
+    fn pill_renders_when_selection_on_last_row() {
+        let area = Rect::new(0, 0, 60, 3);
+        let mut buf = Buffer::empty(area);
+        for y in 0..area.height {
+            for x in 2..area.width.saturating_sub(1) {
+                buf[(x, y)].set_symbol("X");
+            }
+        }
+
+        let mut ui = TranscriptCopyUi::new_with_shortcut(CopySelectionShortcut::CtrlShiftC);
+        ui.render_copy_pill(area, &mut buf, (2, 2), (2, 6), 0, 3);
+
+        let rendered = buf_to_string(&buf, area);
+        assert!(rendered.contains("copy"));
+        assert!(rendered.contains("ctrl + shift + c"));
+
+        let rect = ui.affordance_rect.expect("expected pill to render");
+        assert_eq!(rect.y, 1);
+        assert!(ui.hit_test(rect.x, rect.y));
+    }
 }