fix(helpers): correct sibling lookup in compareDocumentPosition for leaf nodes

src/helpers.spec.ts

@@ -70,6 +70,17 @@ describe("helpers", () => {
                 ),
             ).toBe(10);
         });
+
+        it("reports the correct order when a sibling is a comment", () => {
+            const dom = parseDocument("<div><p></p><!--c--><a></a></div>")
+                .children[0] as Element;
+            const [p, comment, a] = dom.children;
+
+            expect(compareDocumentPosition(p, comment)).toBe(2);
+            expect(compareDocumentPosition(comment, p)).toBe(4);
+            expect(compareDocumentPosition(comment, a)).toBe(2);
+            expect(compareDocumentPosition(a, comment)).toBe(4);
+        });
     });
 
     describe("uniqueSort", () => {
@@ -92,6 +103,19 @@ describe("helpers", () => {
         it("removes duplicate elements", () =>
             expect(uniqueSort([p, a, p])).toStrictEqual([p, a]));
 
+        it("sorts a comment sibling into document order", () => {
+            const withComment = parseDocument(
+                "<div><p></p><!--c--><a></a></div>",
+            ).children[0] as Element;
+            const [firstP, comment, lastA] = withComment.children;
+
+            expect(uniqueSort([lastA, comment, firstP])).toStrictEqual([
+                firstP,
+                comment,
+                lastA,
+            ]);
+        });
+
         it("sorts nodes in document order", () =>
             expect(uniqueSort([a, dom, span, p])).toStrictEqual([
                 dom,

src/helpers.ts

@@ -110,8 +110,8 @@ export function compareDocumentPosition(
 
     const sharedParent = aParents[index - 1];
     const siblings: AnyNode[] = sharedParent.children;
-    const aSibling = aParents[index];
-    const bSibling = bParents[index];
+    const aSibling = index < aParents.length ? aParents[index] : nodeA;
+    const bSibling = index < bParents.length ? bParents[index] : nodeB;
 
     if (siblings.indexOf(aSibling) > siblings.indexOf(bSibling)) {
         if (sharedParent === nodeB) {