fix(tui2): avoid scroll stickiness at cell boundaries (#8695)

Mouse/trackpad scrolling in tui2 applies deltas in visual lines, but the
transcript scroll state was anchored only to CellLine entries.

When a 1-line scroll landed on the synthetic inter-cell Spacer row
(inserted between non-continuation cells),
`TranscriptScroll::anchor_for` would skip that row and snap back to the
adjacent cell line. That makes the resolved top offset unchanged for
small/coalesced scroll deltas, so scrolling appears to get stuck right
before certain cells (commonly user prompts and command output cells).

Fix this by making spacer rows a first-class scroll anchor:
- Add `TranscriptScroll::ScrolledSpacerBeforeCell` and resolve it back
to the spacer row index when present.
- Update `anchor_for`/`scrolled_by` to preserve spacers instead of
skipping them.
- Treat the new variant as "already anchored" in
`lock_transcript_scroll_to_current_view`.

Tests:
- cargo test -p codex-tui2

codex-rs/tui2/src/app.rs

@@ -1135,7 +1135,8 @@ impl App {
         let max_start = total_lines.saturating_sub(max_visible);
         let top_offset = match self.transcript_scroll {
             TranscriptScroll::ToBottom => max_start,
-            TranscriptScroll::Scrolled { .. } => {
+            TranscriptScroll::Scrolled { .. }
+            | TranscriptScroll::ScrolledSpacerBeforeCell { .. } => {
                 // Already anchored; nothing to lock.
                 return;
             }

codex-rs/tui2/src/tui/scrolling.rs

@@ -17,7 +17,7 @@
 //!   the newly flattened line list on the next frame.
 //!
 //! Spacer rows between non-continuation cells are represented as `TranscriptLineMeta::Spacer`.
-//! They are not valid anchors; `anchor_for` will pick the nearest non-spacer line when needed.
+//! They are valid scroll anchors so 1-line scrolling does not "stick" at cell boundaries.
 
 pub(crate) mod mouse;
 pub(crate) use mouse::MouseScrollState;
@@ -78,6 +78,13 @@ pub(crate) enum TranscriptScroll {
         cell_index: usize,
         line_in_cell: usize,
     },
+    /// Anchor the viewport to the spacer row immediately before a cell.
+    ///
+    /// This exists because spacer rows are real, visible transcript rows, and users may scroll
+    /// through them one line at a time (especially with trackpads). Without a dedicated spacer
+    /// anchor, a 1-line scroll that lands on a spacer would snap back to the adjacent cell line
+    /// and appear to "stick" at boundaries.
+    ScrolledSpacerBeforeCell { cell_index: usize },
 }
 
 impl TranscriptScroll {
@@ -108,6 +115,13 @@ impl TranscriptScroll {
                     None => (Self::ToBottom, max_start),
                 }
             }
+            Self::ScrolledSpacerBeforeCell { cell_index } => {
+                let anchor = spacer_before_cell_index(line_meta, cell_index);
+                match anchor {
+                    Some(idx) => (self, idx.min(max_start)),
+                    None => (Self::ToBottom, max_start),
+                }
+            }
         }
     }
 
@@ -142,6 +156,11 @@ impl TranscriptScroll {
             } => anchor_index(line_meta, cell_index, line_in_cell)
                 .unwrap_or(max_start)
                 .min(max_start),
+            Self::ScrolledSpacerBeforeCell { cell_index } => {
+                spacer_before_cell_index(line_meta, cell_index)
+                    .unwrap_or(max_start)
+                    .min(max_start)
+            }
         };
 
         let new_top = if delta_lines < 0 {
@@ -164,15 +183,35 @@ impl TranscriptScroll {
     /// This is the inverse of "resolving a scroll state to a top-row offset":
     /// given a concrete flattened line index, pick a stable `(cell_index, line_in_cell)` anchor.
     ///
-    /// See `resolve_top` for `line_meta` semantics. This prefers the nearest line at or after `start`
-    /// (skipping spacer rows), falling back to the nearest line before it when needed.
+    /// See `resolve_top` for `line_meta` semantics. This prefers the line at `start` (including
+    /// spacer rows), falling back to the nearest non-spacer line after or before it when needed.
     pub(crate) fn anchor_for(line_meta: &[TranscriptLineMeta], start: usize) -> Option<Self> {
-        let anchor =
-            anchor_at_or_after(line_meta, start).or_else(|| anchor_at_or_before(line_meta, start));
-        anchor.map(|(cell_index, line_in_cell)| Self::Scrolled {
-            cell_index,
-            line_in_cell,
-        })
+        if line_meta.is_empty() {
+            return None;
+        }
+
+        let start = start.min(line_meta.len().saturating_sub(1));
+        match line_meta[start] {
+            TranscriptLineMeta::CellLine {
+                cell_index,
+                line_in_cell,
+            } => Some(Self::Scrolled {
+                cell_index,
+                line_in_cell,
+            }),
+            TranscriptLineMeta::Spacer => {
+                if let Some((cell_index, _)) = anchor_at_or_after(line_meta, start) {
+                    Some(Self::ScrolledSpacerBeforeCell { cell_index })
+                } else {
+                    anchor_at_or_before(line_meta, start).map(|(cell_index, line_in_cell)| {
+                        Self::Scrolled {
+                            cell_index,
+                            line_in_cell,
+                        }
+                    })
+                }
+            }
+        }
     }
 }
 
@@ -198,6 +237,26 @@ fn anchor_index(
         })
 }
 
+/// Locate the flattened line index for the spacer row immediately before `cell_index`.
+///
+/// The spacer itself is not uniquely tagged in `TranscriptLineMeta`, so we locate the first
+/// visual line of the cell (`line_in_cell == 0`) and, if it is preceded by a spacer row, return
+/// that spacer's index. If the spacer is missing (for example when the cell is a stream
+/// continuation), we fall back to the cell's first line index so scrolling remains usable.
+fn spacer_before_cell_index(line_meta: &[TranscriptLineMeta], cell_index: usize) -> Option<usize> {
+    let cell_first = anchor_index(line_meta, cell_index, 0)?;
+    if cell_first > 0
+        && matches!(
+            line_meta.get(cell_first.saturating_sub(1)),
+            Some(TranscriptLineMeta::Spacer)
+        )
+    {
+        Some(cell_first.saturating_sub(1))
+    } else {
+        Some(cell_first)
+    }
+}
+
 /// Find the first transcript line at or after the given flattened index.
 fn anchor_at_or_after(line_meta: &[TranscriptLineMeta], start: usize) -> Option<(usize, usize)> {
     if line_meta.is_empty() {
@@ -272,6 +331,33 @@ mod tests {
         assert_eq!(top, 2);
     }
 
+    #[test]
+    fn scrolled_by_can_land_on_spacer_rows() {
+        let meta = meta(&[
+            cell_line(0, 0),
+            TranscriptLineMeta::Spacer,
+            cell_line(1, 0),
+            cell_line(1, 1),
+        ]);
+
+        let scroll = TranscriptScroll::Scrolled {
+            cell_index: 1,
+            line_in_cell: 0,
+        };
+
+        assert_eq!(
+            scroll.scrolled_by(-1, &meta, 2),
+            TranscriptScroll::ScrolledSpacerBeforeCell { cell_index: 1 }
+        );
+        assert_eq!(
+            TranscriptScroll::ScrolledSpacerBeforeCell { cell_index: 1 }.scrolled_by(-1, &meta, 2),
+            TranscriptScroll::Scrolled {
+                cell_index: 0,
+                line_in_cell: 0
+            }
+        );
+    }
+
     #[test]
     fn resolve_top_scrolled_falls_back_when_anchor_missing() {
         let meta = meta(&[cell_line(0, 0), TranscriptLineMeta::Spacer, cell_line(1, 0)]);
@@ -350,17 +436,11 @@ mod tests {
 
         assert_eq!(
             TranscriptScroll::anchor_for(&meta, 0),
-            Some(TranscriptScroll::Scrolled {
-                cell_index: 0,
-                line_in_cell: 0
-            })
+            Some(TranscriptScroll::ScrolledSpacerBeforeCell { cell_index: 0 })
         );
         assert_eq!(
             TranscriptScroll::anchor_for(&meta, 2),
-            Some(TranscriptScroll::Scrolled {
-                cell_index: 1,
-                line_in_cell: 0
-            })
+            Some(TranscriptScroll::ScrolledSpacerBeforeCell { cell_index: 1 })
         );
         assert_eq!(
             TranscriptScroll::anchor_for(&meta, 3),