fix: preserve complete alternate-screen history reads

src/terminal/history_read.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use crate::ghostty::{CellWide, ScreenTextRow};
 use crate::pane::TerminalReadSnapshot;
 
@@ -54,30 +56,27 @@ pub(crate) fn merge_scrolled_up(
     if previous_text == next_text {
         return UpwardMerge::Unchanged;
     }
-    let Some(shift) = best_upward_shift(&previous_text, &next_text) else {
-        return UpwardMerge::Unaligned;
-    };
-    let Some(boundary) = (0..previous_text.len().saturating_sub(shift)).find_map(|index| {
-        let next_index = index + shift;
-        (!previous_text[index].is_empty() && previous_text[index] == next_text[next_index])
-            .then_some(next_index)
-    }) else {
+    let Some((shift, anchor)) = upward_alignment(&previous_text, &next_text) else {
         return UpwardMerge::Unaligned;
     };
-    let added: Vec<_> = next.rows[..boundary]
+    let history_text = row_identities(history);
+    // A retained fragment may lack a complete scrollbar track/thumb pattern;
+    // keep exact text matching alongside normalized identities for that case.
+    let anchor_text = row_text(&previous.rows[anchor]);
+    let Some(history_anchor) = history_text
         .iter()
-        .enumerate()
-        .filter(|(index, _)| {
-            next_text[*index].is_empty() || previous_text.get(*index) != Some(&next_text[*index])
-        })
-        .map(|(_, row)| row.clone())
-        .collect();
-    if added.is_empty() {
+        .take(previous.rows.len())
+        .position(|text| text == &previous_text[anchor] || text.as_str() == anchor_text.trim_end())
+    else {
         return UpwardMerge::Unaligned;
+    };
+    let boundary = anchor + shift;
+    // Refresh the overlap as well as new rows: a pinned header may have obscured
+    // transcript text at the top of the previous viewport.
+    history.splice(0..history_anchor, next.rows[..boundary].iter().cloned());
+    UpwardMerge::Advanced {
+        rows: boundary.saturating_sub(history_anchor),
     }
-    let rows = added.len();
-    history.splice(0..0, added);
-    UpwardMerge::Advanced { rows }
 }
 
 pub(crate) fn snapshot_text(
@@ -96,12 +95,21 @@ pub(crate) fn snapshot_text(
     TerminalReadSnapshot { text, truncated }
 }
 
-fn best_upward_shift(previous: &[String], next: &[String]) -> Option<usize> {
-    let mut best = None;
+fn upward_alignment(previous: &[String], next: &[String]) -> Option<(usize, usize)> {
+    let mut previous_counts = HashMap::new();
+    let mut next_counts = HashMap::new();
+    for text in previous {
+        *previous_counts.entry(text.as_str()).or_insert(0usize) += 1;
+    }
+    for text in next {
+        *next_counts.entry(text.as_str()).or_insert(0usize) += 1;
+    }
+    let mut alignment = None;
     for shift in 1..previous.len() {
         let overlap = previous.len() - shift;
         let mut comparable = 0usize;
         let mut matches = 0usize;
+        let mut first_anchor = None;
         for index in 0..overlap {
             let before = &previous[index];
             let after = &next[index + shift];
@@ -111,26 +119,59 @@ fn best_upward_shift(previous: &[String], next: &[String]) -> Option<usize> {
             comparable += 1;
             if before == after {
                 matches += 1;
+                if previous_counts.get(before.as_str()) == Some(&1)
+                    && next_counts.get(after.as_str()) == Some(&1)
+                {
+                    first_anchor.get_or_insert(index);
+                }
             }
         }
-        if comparable == 0
-            || matches.saturating_mul(100) < comparable.saturating_mul(MIN_ALIGNMENT_RATIO_PERCENT)
-        {
+        // Repeated rows alone cannot distinguish a small scroll from a larger one.
+        let Some(anchor) = first_anchor else {
+            continue;
+        };
+        if matches.saturating_mul(100) < comparable.saturating_mul(MIN_ALIGNMENT_RATIO_PERCENT) {
             continue;
         }
-        if best.is_none_or(|(_, best_matches, best_comparable)| {
-            matches > best_matches || (matches == best_matches && comparable > best_comparable)
-        }) {
-            best = Some((shift, matches, comparable));
+        // A row unique in each viewport can still occur elsewhere in the transcript.
+        // Competing plausible shifts are ambiguous, not votes for the best score.
+        if alignment.is_some() {
+            return None;
         }
+        alignment = Some((shift, anchor));
     }
-    best.map(|(shift, _, _)| shift)
+    alignment
 }
 
 fn row_identities(rows: &[ScreenTextRow]) -> Vec<String> {
-    rows.iter()
+    let mut identities: Vec<_> = rows
+        .iter()
         .map(|row| row_text(row).trim_end().to_string())
-        .collect()
+        .collect();
+    let mut start = 0;
+    while start < rows.len() {
+        let mut end = start;
+        let mut track = false;
+        let mut thumb = false;
+        while end < rows.len() && rows[end].cells.len() == rows[start].cells.len() {
+            match rows[end].cells.last().map(|cell| cell.graphemes.as_slice()) {
+                Some([0x2502]) => track = true,
+                Some([0x2503 | 0x2588]) => thumb = true,
+                _ => break,
+            }
+            end += 1;
+        }
+        // Require a vertical track and thumb, not an isolated border character.
+        // Normalize comparison keys only; retained terminal cells stay untouched.
+        if end - start >= 3 && track && thumb {
+            for identity in &mut identities[start..end] {
+                identity.pop();
+                identity.truncate(identity.trim_end().len());
+            }
+        }
+        start = end.max(start + 1);
+    }
+    identities
 }
 
 fn wrapped_text(rows: &[ScreenTextRow]) -> String {
@@ -257,7 +298,164 @@ mod tests {
         );
         assert_eq!(
             row_identities(&history),
-            ["line 2", "line 3", "sticky", "line 4", "line 5", "line 6", "line 7"]
+            ["sticky", "line 2", "line 3", "line 4", "line 5", "line 6", "line 7"]
+        );
+    }
+
+    #[test]
+    fn later_overlap_recovers_text_obscured_by_a_pinned_header() {
+        let initial = snapshot(&["line 4", "line 5", "line 6", "line 7", "line 8"]);
+        let scrolled = snapshot(&["pinned", "line 3", "line 4", "line 5", "line 6"]);
+        let older = snapshot(&["pinned", "line 1", "line 2", "line 3", "line 4"]);
+        let top = snapshot(&["title", "line 0", "line 1", "line 2", "line 3"]);
+        let mut history = initial.rows.clone();
+        for (previous, next) in [(&initial, &scrolled), (&scrolled, &older), (&older, &top)] {
+            assert!(matches!(
+                merge_scrolled_up(&mut history, previous, next),
+                UpwardMerge::Advanced { .. }
+            ));
+        }
+        assert_eq!(
+            row_identities(&history),
+            [
+                "title", "line 0", "line 1", "line 2", "line 3", "line 4", "line 5", "line 6",
+                "line 7", "line 8"
+            ]
+        );
+    }
+
+    #[test]
+    fn scrolling_preserves_repeated_continuation_rows() {
+        let rows: Vec<_> = (1..=18)
+            .flat_map(|number| {
+                [
+                    row(&format!("line {number:03} begins")),
+                    row("same continuation"),
+                    row("same ending"),
+                ]
+            })
+            .collect();
+        for shift in [3, 15] {
+            for offset in 0..3 {
+                let previous = ScreenSnapshot {
+                    cols: 20,
+                    rows: rows[offset + shift..offset + shift + 36].to_vec(),
+                };
+                let next = ScreenSnapshot {
+                    cols: 20,
+                    rows: rows[offset..offset + 36].to_vec(),
+                };
+                let mut history = previous.rows.clone();
+
+                assert_eq!(
+                    merge_scrolled_up(&mut history, &previous, &next),
+                    UpwardMerge::Advanced { rows: shift },
+                    "shift={shift}, offset={offset}"
+                );
+                assert_eq!(&history[..shift], &next.rows[..shift]);
+                assert_eq!(&history[shift..], &previous.rows);
+            }
+        }
+    }
+
+    fn transcript(first: usize, scrollbar: bool) -> ScreenSnapshot {
+        let mut rows: Vec<_> = (first..first + 10)
+            .enumerate()
+            .map(|(index, number)| {
+                let edge = if !scrollbar {
+                    ' '
+                } else if (3..6).contains(&index) {
+                    '┃'
+                } else {
+                    '│'
+                };
+                row(&format!("{:<19}{edge}", format!("line {number}")))
+            })
+            .collect();
+        rows.extend([row("prompt"), row("status")]);
+        ScreenSnapshot { cols: 20, rows }
+    }
+
+    #[test]
+    fn scrolling_with_an_appearing_scrollbar_recovers_history_and_preserves_cells() {
+        let initial = transcript(10, false);
+        let scrolled = transcript(7, true);
+        let mut history = initial.rows.clone();
+
+        assert_eq!(
+            merge_scrolled_up(&mut history, &initial, &scrolled),
+            UpwardMerge::Advanced { rows: 3 }
+        );
+        assert_eq!(&history[..3], &scrolled.rows[..3]);
+        assert_eq!(&history[3..], &initial.rows);
+        assert!(initial.similar_text(&transcript(10, true)));
+        assert!(!initial.similar_text(&scrolled));
+
+        let mut older = transcript(4, true);
+        for line in &mut older.rows[..10] {
+            let edge = line.cells.last_mut().unwrap();
+            edge.graphemes = if edge.graphemes == ['│' as u32] {
+                vec!['█' as u32]
+            } else {
+                vec!['│' as u32]
+            };
+        }
+        assert_eq!(
+            merge_scrolled_up(&mut history, &scrolled, &older),
+            UpwardMerge::Advanced { rows: 3 }
+        );
+        assert_eq!(&history[..3], &older.rows[..3]);
+    }
+
+    #[test]
+    fn chained_scroll_matches_a_retained_anchor_after_the_scrollbar_thumb_moves() {
+        let initial = transcript(10, true);
+        let mut scrolled = transcript(7, true);
+        let mut older = transcript(4, true);
+        for line in &mut scrolled.rows[..3] {
+            *line = row(&format!("{:<19}│", "repeated"));
+        }
+        for line in &mut older.rows[3..6] {
+            *line = row(&format!("{:<19}┃", "repeated"));
+        }
+        let mut history = initial.rows.clone();
+        assert_eq!(
+            merge_scrolled_up(&mut history, &initial, &scrolled),
+            UpwardMerge::Advanced { rows: 3 }
+        );
+        assert_eq!(
+            merge_scrolled_up(&mut history, &scrolled, &older),
+            UpwardMerge::Advanced { rows: 3 }
+        );
+        assert_eq!(&history[..6], &older.rows[..6]);
+        assert_eq!(&history[6..], &initial.rows);
+    }
+
+    #[test]
+    fn scrollbar_normalization_does_not_hide_real_edge_text_or_align_unrelated_output() {
+        let initial = transcript(10, false);
+        let mut changed = transcript(10, true);
+        for line in &mut changed.rows[..10] {
+            line.cells[18].graphemes = vec!['x' as u32];
+        }
+        assert!(!initial.similar_text(&changed));
+        let mut history = initial.rows.clone();
+        assert_eq!(
+            merge_scrolled_up(&mut history, &initial, &changed),
+            UpwardMerge::Unaligned
+        );
+        assert_eq!(history, initial.rows);
+
+        let mut real_edge = initial.clone();
+        for line in &mut real_edge.rows[..10] {
+            line.cells[19].graphemes = vec!['x' as u32];
+        }
+        assert!(!real_edge.similar_text(&transcript(10, true)));
+
+        let boxed = snapshot(&["one   │", "two   │", "three │", "four  │"]);
+        assert_eq!(
+            row_identities(&boxed.rows),
+            ["one   │", "two   │", "three │", "four  │"]
         );
     }
 
@@ -281,6 +479,46 @@ mod tests {
         assert_eq!(history, previous.rows);
     }
 
+    #[test]
+    fn history_anchor_matches_text_despite_different_wrap_metadata() {
+        let previous = snapshot(&["line 3", "line 4", "line 5", "status"]);
+        let next = snapshot(&["line 1", "line 2", "line 3", "line 4"]);
+        let mut history = previous.rows.clone();
+        history[0].wrap_continuation = true;
+        assert_eq!(
+            merge_scrolled_up(&mut history, &previous, &next),
+            UpwardMerge::Advanced { rows: 2 }
+        );
+        assert_eq!(
+            row_identities(&history),
+            ["line 1", "line 2", "line 3", "line 4", "line 5", "status"]
+        );
+    }
+
+    #[test]
+    fn competing_unique_anchors_do_not_prove_scroll_distance() {
+        let previous = snapshot(&["U1", "U2", "U3", "c", "d"]);
+        let next = snapshot(&["a", "b", "U2", "U3", "U1"]);
+        let mut history = previous.rows.clone();
+        assert_eq!(
+            merge_scrolled_up(&mut history, &previous, &next),
+            UpwardMerge::Unaligned
+        );
+        assert_eq!(history, previous.rows);
+    }
+
+    #[test]
+    fn repeated_rows_alone_do_not_prove_scroll_distance() {
+        let previous = snapshot(&["a", "b", "a", "b", "a", "b"]);
+        let next = snapshot(&["b", "a", "b", "a", "b", "a"]);
+        let mut history = previous.rows.clone();
+        assert_eq!(
+            merge_scrolled_up(&mut history, &previous, &next),
+            UpwardMerge::Unaligned
+        );
+        assert_eq!(history, previous.rows);
+    }
+
     #[test]
     fn snapshot_text_limits_rendered_rows_before_unwrapping() {
         let mut first = row("hello ");