renderdag: emit link_line for same-column named parents

eden/scm/lib/renderdag/src/ascii.rs

@@ -240,6 +240,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn basic_disconnected() {
+        assert_eq!(
+            render(&test_fixtures::BASIC_DISCONNECTED),
+            r#"
+            o  C
+            |
+            o  B
+
+            o  A"#
+        );
+    }
+
+    #[test]
+    fn basic_disconnected_min_row_height_1() {
+        let mut renderer = GraphRowRenderer::new()
+            .output()
+            .with_min_row_height(1)
+            .build_ascii();
+        assert_eq!(
+            render_string(&test_fixtures::BASIC_DISCONNECTED, &mut renderer),
+            r#"
+            o  C
+            |
+            o  B
+            o  A"#
+        );
+    }
+
     #[test]
     fn branches_and_merges() {
         assert_eq!(

eden/scm/lib/renderdag/src/ascii_large.rs

@@ -296,6 +296,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn basic_disconnected() {
+        assert_eq!(
+            render(&test_fixtures::BASIC_DISCONNECTED),
+            r#"
+            o  C
+            |
+            |
+            o  B
+
+
+            o  A"#
+        );
+    }
+
     #[test]
     fn branches_and_merges() {
         assert_eq!(

eden/scm/lib/renderdag/src/box_drawing.rs

@@ -319,6 +319,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn basic_disconnected() {
+        assert_eq!(
+            render(&test_fixtures::BASIC_DISCONNECTED),
+            r#"
+            o  C
+            │
+            o  B
+
+            o  A"#
+        );
+    }
+
+    #[test]
+    fn basic_disconnected_min_row_height_1() {
+        let mut renderer = GraphRowRenderer::new()
+            .output()
+            .with_min_row_height(1)
+            .build_box_drawing();
+        assert_eq!(
+            render_string(&test_fixtures::BASIC_DISCONNECTED, &mut renderer),
+            r#"
+            o  C
+            │
+            o  B
+            o  A"#
+        );
+    }
+
     #[test]
     fn branches_and_merges() {
         assert_eq!(

eden/scm/lib/renderdag/src/render.rs

@@ -518,6 +518,9 @@ where
                 } else if i == column {
                     link_line[i] |= LinkLine::CHILD
                         | p.to_link_line(LinkLine::VERT_PARENT, LinkLine::VERT_ANCESTOR);
+                    if p.id().is_some() {
+                        need_link_line = true;
+                    }
                 } else {
                     link_line[i] |=
                         p.to_link_line(LinkLine::LEFT_FORK_PARENT, LinkLine::LEFT_FORK_ANCESTOR);

eden/scm/lib/renderdag/src/test_fixtures.rs

@@ -23,6 +23,18 @@ pub(crate) const BASIC: TestFixture = TestFixture {
     missing: &[],
 };
 
+// A-B is a connected pair; C is an isolated node that ends up in the same
+// column after A's column is freed. Used to verify that the renderer
+// visually distinguishes connected from unconnected adjacent nodes.
+pub(crate) const BASIC_DISCONNECTED: TestFixture = TestFixture {
+    dag: "A B-C",
+    messages: &[],
+    heads: &["A", "C"],
+    reserve: &[],
+    ancestors: &[],
+    missing: &[],
+};
+
 pub(crate) const BRANCHES_AND_MERGES: TestFixture = TestFixture {
     dag: r#"
                       T /---------------N--O---\           T