feat: show base-relative changes in conflict panes

tests/unit/diff.rs

@@ -19,7 +19,8 @@ fn conflict_history_preserves_source_gaps_base_changes_and_native_eof_rows() {
     assert_eq!(
         rows,
         [
-            [Some(0), Some(0), Some(0)],
+            [None, Some(0), None],
+            [Some(0), None, Some(0)],
             [None, None, Some(1)],
             [Some(1), Some(1), Some(2)],
             [Some(2), Some(2), Some(3)],
@@ -44,13 +45,13 @@ fn conflict_history_preserves_source_gaps_base_changes_and_native_eof_rows() {
         assert!(line.changes().any(|range| range.contains(&column)));
     }
     assert!(!line.changes().any(|range| range.contains(&4)));
-    let ColumnRow::Text(line) = base.row(2).unwrap() else {
+    let ColumnRow::Text(line) = base.row(3).unwrap() else {
         panic!("unchanged base row must contain text");
     };
     assert!(!line.changed);
     assert_eq!(line.changes().count(), 0);
     let right = history.column(2);
-    let ColumnRow::Text(line) = right.view().row(1).unwrap() else {
+    let ColumnRow::Text(line) = right.view().row(2).unwrap() else {
         panic!("inserted right row must contain text");
     };
     assert_eq!(line.text, Some("added"));
@@ -62,9 +63,45 @@ fn conflict_history_preserves_source_gaps_base_changes_and_native_eof_rows() {
         "abc deg\nadded\nkeep\n",
         "xbc def\nkeep\n",
     ]);
-    assert_eq!(history.column(1).view().native_row(1), Some(1));
-    assert_eq!(history.column(0).view().native_row(1), None);
-    assert_eq!(history.column(2).view().native_row(1), None);
+    assert_eq!(history.column(1).view().native_row(2), Some(1));
+    assert_eq!(history.column(0).view().native_row(2), None);
+    assert_eq!(history.column(2).view().native_row(2), None);
+    // Deletions are visible on either side, but never become editable source rows.
+    let history = HistoryComparison::new(["keep\n", "old\nkeep\n", "new\nkeep\n"]);
+    for column in [0, 2] {
+        let projection = history.column(column).view();
+        let ColumnRow::Text(line) = projection.row(0).unwrap() else {
+            panic!("removed base text must remain visible");
+        };
+        assert_eq!(line.text, Some("old"));
+        assert!(line.changed);
+        assert_eq!(line.polarity, Focus::Before);
+        assert!(!line.native);
+        assert_eq!(projection.native_row(0), None);
+        assert_eq!(projection.text_row(0), Some((usize::MAX, "old")));
+    }
+    assert!(matches!(
+        history.column(0).view().row(1),
+        Some(ColumnRow::Gap)
+    ));
+    let right = history.column(2).view();
+    let ColumnRow::Text(line) = right.row(1).unwrap() else {
+        panic!("replacement must be an addition");
+    };
+    assert_eq!(line.text, Some("new"));
+    assert_eq!(line.polarity, Focus::After);
+    assert_eq!(right.native_row(1), Some(0));
+    assert_eq!(right.display_row(0), Some(1));
+    let history = HistoryComparison::new(["left", "", "right"]);
+    for column in [0, 2] {
+        let projection = history.column(column).view();
+        let ColumnRow::Text(line) = projection.row(0).unwrap() else {
+            panic!("add/add without a base must show additions on both sides");
+        };
+        assert!(line.changed && line.native);
+        assert_eq!(line.polarity, Focus::After);
+        assert_eq!(projection.native_row(0), Some(0));
+    }
     for text in ["", "\n", "same", "same\n"] {
         let history = HistoryComparison::new([text; 3]);
         let native_rows = text.split('\n').count();
@@ -83,7 +120,7 @@ fn resolver_equal_text_retains_native_rows_and_cached_viewport() {
     for text in ["", "\n", "same\n", "same"] {
         let lines: Vec<_> = text.split('\n').map(str::to_owned).collect();
         let mut state = DiffState::new();
-        state.set_resolver_text(&lines, &lines);
+        state.set_resolver_text(&lines, &lines, None);
         assert_eq!(state.pane_layout(), PaneLayout::Split);
         assert!(state.comparison.as_ref().unwrap().files[0].hunks.is_empty());
         assert_eq!(
@@ -99,7 +136,7 @@ fn resolver_equal_text_retains_native_rows_and_cached_viewport() {
         state.top = 1;
         state.left_before = 3;
         state.left_after = 4;
-        state.set_resolver_text(&lines, &lines);
+        state.set_resolver_text(&lines, &lines, None);
         assert_eq!(state.comparison_id, id);
         assert_eq!((state.top, state.left_before, state.left_after), (1, 3, 4));
     }
@@ -2407,7 +2444,7 @@ fn column_view_distinguishes_native_terminal_rows_from_missing_ordinary_text() {
     for text in ["", "same\n"] {
         let lines: Vec<_> = text.split('\n').map(str::to_owned).collect();
         let mut state = DiffState::new();
-        state.set_resolver_text(&lines, &lines);
+        state.set_resolver_text(&lines, &lines, None);
         for focus in [Focus::Before, Focus::After] {
             let view = state.column_view(focus).unwrap();
             let native = lines.len() - 1;