fix(diff): a new or deleted file draws unified in the split view (#1138)

A new or deleted file has one side only, so in the split view half its card was empty. Such a file now draws unified whatever the view, and the others still split. A drag takes its mode from the row it starts on, so selecting and copying in one of these files works in either view. `file_mode` in `diff_list.rs` picks the mode, with tests; `cargo test --bin tty7-app diff` gives 90 passed, 1 ignored.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/diff_list.rs

@@ -155,7 +155,17 @@ pub(crate) fn preview_rows(file: &FileDiff, mode: DiffViewMode) -> Vec<DiffRow>
     file_rows(usize::MAX, file, true, mode)
 }
 
+/// The view `file` draws in: a new or deleted file has one side only, so
+/// split would be half empty.
+pub(crate) fn file_mode(file: &FileDiff, mode: DiffViewMode) -> DiffViewMode {
+    match file.status {
+        FileStatus::Added | FileStatus::Deleted => DiffViewMode::Unified,
+        _ => mode,
+    }
+}
+
 fn file_rows(index: usize, file: &FileDiff, expanded: bool, mode: DiffViewMode) -> Vec<DiffRow> {
+    let mode = file_mode(file, mode);
     let expandable =
         !file.binary && (!file.hunks.is_empty() || file.truncated == Some(Truncation::Budget));
     let shown_path = match &file.old_path {
@@ -382,6 +392,34 @@ mod tests {
         );
     }
 
+    /// A new or deleted file has nothing to set beside it, so split draws it
+    /// unified; a modified file in the same list still splits.
+    #[test]
+    fn split_draws_a_new_or_deleted_file_unified() {
+        let added = FileDiff {
+            status: FileStatus::Added,
+            ..file("new.rs", 1)
+        };
+        let deleted = FileDiff {
+            status: FileStatus::Deleted,
+            ..file("gone.rs", 1)
+        };
+        for one_sided in [added, deleted] {
+            let snap = snapshot(vec![one_sided, file("a.rs", 1)]);
+            let rows = build_rows(&snap, &open(["new.rs"]), None, DiffViewMode::Split, false);
+            let kinds: Vec<_> = shape(&rows)
+                .into_iter()
+                .filter(|k| matches!(*k, "split" | "unified"))
+                .collect();
+            assert_eq!(
+                kinds,
+                [
+                    "unified", "unified", "unified", "unified", "split", "split", "split"
+                ]
+            );
+        }
+    }
+
     #[test]
     fn only_the_first_hunk_of_a_file_follows_its_header() {
         let rows = build_rows(

src/ui/diff_overlay.rs

@@ -1046,16 +1046,6 @@ impl Tty7App {
         let active = self.active;
         let overlay = self.tabs.get_mut(active)?.diff_overlay.as_mut()?;
 
-        // Forget a range the rows under it no longer answer to. The two views
-        // pair the same lines differently, so a row range drawn in one of them
-        // points at other code in the other. Here rather than beside the
-        // switch that flips the mode: this is the one place that knows which
-        // rows are about to be drawn.
-        if overlay.selection.as_ref().is_some_and(|s| s.mode != mode) {
-            overlay.selection = None;
-            overlay.selecting = false;
-        }
-
         let snap = match &overlay.load {
             DiffLoad::Loading => return Some(DiffBody::Message(t(L10nKey::DiffReading))),
             DiffLoad::NotARepo => return Some(DiffBody::Message(t(L10nKey::DiffNotARepo))),
@@ -1081,6 +1071,27 @@ impl Tty7App {
             None => None,
         };
 
+        // Forget a range the rows under it no longer answer to. The two views
+        // pair the same lines differently, so a row range drawn in one of them
+        // points at other code in the other. Here rather than beside the
+        // switch that flips the mode: this is the one place that knows which
+        // rows are about to be drawn.
+        let drawn = |path: &str| {
+            preview
+                .as_deref()
+                .filter(|f| f.path == path)
+                .or_else(|| snap.files.iter().find(|f| f.path == path))
+                .map_or(mode, |f| crate::ui::diff_list::file_mode(f, mode))
+        };
+        if overlay
+            .selection
+            .as_ref()
+            .is_some_and(|s| s.mode != drawn(&s.path))
+        {
+            overlay.selection = None;
+            overlay.selecting = false;
+        }
+
         let focused = focused_file(&snap, overlay);
         let from = RowsFrom {
             snap: &snap,
@@ -1135,7 +1146,6 @@ impl Tty7App {
         let drag = Drag {
             sel: overlay.selection.clone().map(Rc::new),
             selecting: overlay.selecting,
-            mode: view_mode(cx),
         };
         let body = gpui::list(list.clone(), move |ix, _window, cx| {
             #[cfg(test)]
@@ -1197,9 +1207,6 @@ struct Drag {
     /// while one is: a diff runs to thousands of rows, and a listener each is
     /// worth paying for during a drag and not otherwise.
     selecting: bool,
-    /// The view the rows on screen are drawn in, which is the view a press
-    /// starts its selection in.
-    mode: DiffViewMode,
 }
 
 impl Drag {
@@ -1535,7 +1542,12 @@ fn diff_row_drag<E: InteractiveElement + Styled>(
     drag: &Drag,
     app: &gpui::WeakEntity<Tty7App>,
 ) -> E {
-    let mode = drag.mode;
+    // From the row, not the overlay's view: a new or deleted file draws
+    // unified whatever the view (`diff_list::file_mode`).
+    let mode = match side {
+        Some(_) => DiffViewMode::Split,
+        None => DiffViewMode::Unified,
+    };
     // The I-beam is the only standing sign that this text can be taken;
     // nothing else about a row says so until one is dragged.
     let el = el.cursor_text().on_mouse_down(MouseButton::Left, {