Prefer additions when resolving hunk comments

Author: benvinegar

src/core/liveComments.ts

@@ -43,6 +43,7 @@ export function findHunkIndexForLine(file: DiffFile, side: DiffSide, line: numbe
 export function firstCommentTargetForHunk(hunk: Hunk): Omit<ResolvedCommentTarget, "hunkIndex"> {
   let deletionLineNumber = hunk.deletionStart;
   let additionLineNumber = hunk.additionStart;
+  let firstDeletionLine: number | undefined;
 
   for (const content of hunk.hunkContent) {
     if (content.type === "context") {
@@ -58,12 +59,19 @@ export function firstCommentTargetForHunk(hunk: Hunk): Omit<ResolvedCommentTarge
       };
     }
 
-    if (content.deletions > 0) {
-      return {
-        side: "old",
-        line: deletionLineNumber,
-      };
+    if (content.deletions > 0 && firstDeletionLine === undefined) {
+      firstDeletionLine = deletionLineNumber;
     }
+
+    deletionLineNumber += content.deletions;
+    additionLineNumber += content.additions;
+  }
+
+  if (firstDeletionLine !== undefined) {
+    return {
+      side: "old",
+      line: firstDeletionLine,
+    };
   }
 
   const fallbackRange = hunkLineRange(hunk);

test/live-comments.test.ts

@@ -4,6 +4,7 @@ import {
   buildLiveComment,
   findDiffFileByPath,
   findHunkIndexForLine,
+  firstCommentTargetForHunk,
   hunkLineRange,
   resolveCommentTarget,
 } from "../src/core/liveComments";
@@ -125,6 +126,25 @@ describe("live comment helpers", () => {
     });
   });
 
+  test("prefers a later addition over an earlier deletion-only chunk", () => {
+    const target = firstCommentTargetForHunk({
+      additionStart: 20,
+      additionLines: 1,
+      deletionStart: 20,
+      deletionLines: 1,
+      hunkContent: [
+        { type: "change", deletions: 1, additions: 0 },
+        { type: "context", lines: 2 },
+        { type: "change", deletions: 0, additions: 1 },
+      ],
+    } as Parameters<typeof firstCommentTargetForHunk>[0]);
+
+    expect(target).toEqual({
+      side: "new",
+      line: 22,
+    });
+  });
+
   test("builds a live MCP comment annotation", () => {
     const comment = buildLiveComment(
       {