1 files changed, 98 insertions, 0 deletions
diff --git a/src/client/shell/scroll.rs b/src/client/shell/scroll.rs
new file mode 100644
index 00000000..a0496cb8
--- /dev/null
+++ b/src/client/shell/scroll.rs
@@ -0,0 +1,98 @@
+use ratatui::{buffer::Buffer, layout::Rect, style::Style};
+
+use super::Palette;
+
+pub(super) fn list_scroll_metrics(
+    row_heights: &[u16],
+    gaps_after: &[u16],
+    body_height: u16,
+    requested_start: usize,
+) -> crate::pane::ScrollMetrics {
+    if row_heights.is_empty() || body_height == 0 {
+        return crate::pane::ScrollMetrics {
+            offset_from_bottom: 0,
+            max_offset_from_bottom: 0,
+            viewport_rows: 0,
+        };
+    }
+
+    let mut used = 0u16;
+    let mut max_start = row_heights.len();
+    for index in (0..row_heights.len()).rev() {
+        let height = row_heights[index].max(1).min(body_height);
+        let gap = gaps_after.get(index).copied().unwrap_or(0);
+        if used.saturating_add(height).saturating_add(gap) > body_height {
+            break;
+        }
+        used = used.saturating_add(height).saturating_add(gap);
+        max_start = index;
+    }
+    max_start = max_start.min(row_heights.len().saturating_sub(1));
+    let start = requested_start.min(max_start);
+
+    let mut viewport_rows = 0usize;
+    let mut used = 0u16;
+    for (index, row_height) in row_heights.iter().enumerate().skip(start) {
+        let height = (*row_height).max(1).min(body_height);
+        if used.saturating_add(height) > body_height {
+            break;
+        }
+        used = used.saturating_add(height);
+        viewport_rows += 1;
+        let gap = gaps_after.get(index).copied().unwrap_or(0);
+        if used.saturating_add(gap) > body_height {
+            break;
+        }
+        used = used.saturating_add(gap);
+    }
+
+    crate::pane::ScrollMetrics {
+        offset_from_bottom: max_start.saturating_sub(start),
+        max_offset_from_bottom: max_start,
+        viewport_rows,
+    }
+}
+
+pub(super) fn render_list_scrollbar(
+    buffer: &mut Buffer,
+    track: Rect,
+    metrics: crate::pane::ScrollMetrics,
+    palette: &Palette,
+) {
+    let Some(thumb) = crate::ui::scrollbar_thumb(metrics, track) else {
+        return;
+    };
+    for row in track.y..track.bottom() {
+        if let Some(cell) = buffer.cell_mut((track.x, row)) {
+            cell.set_symbol("▕")
+                .set_style(Style::default().fg(palette.surface_dim));
+        }
+    }
+    for row in thumb.top..thumb.top.saturating_add(thumb.len) {
+        if let Some(cell) = buffer.cell_mut((track.x, row)) {
+            cell.set_symbol("▕")
+                .set_style(Style::default().fg(palette.overlay0));
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn list_metrics_preserve_variable_rows_and_caller_owned_gap_policy() {
+        let top = list_scroll_metrics(&[1, 3, 2], &[1, 1, 0], 5, 0);
+        assert_eq!(top.max_offset_from_bottom, 2);
+        assert_eq!(top.offset_from_bottom, 2);
+        assert_eq!(top.viewport_rows, 2);
+
+        let bottom = list_scroll_metrics(&[1, 3, 2], &[1, 1, 0], 5, usize::MAX);
+        assert_eq!(bottom.offset_from_bottom, 0);
+        assert_eq!(bottom.viewport_rows, 1);
+
+        let parent_child = list_scroll_metrics(&[2, 2, 2], &[0, 1, 0], 5, 0);
+        assert_eq!(parent_child.max_offset_from_bottom, 1);
+        assert_eq!(parent_child.viewport_rows, 2);
+    }
+}