fix: diff PRs against the up-to-date base to avoid unrelated changes (#27)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: ymansurozer

src/cli.ts

@@ -309,7 +309,18 @@ async function resolvePrRef(ref: string, root: string): Promise<{ head: string;
   if (!info.headRefName || !info.baseRefName)
     throw new Error(`PR "${ref}" did not resolve to a head/base branch.`);
   await gh(["pr", "checkout", ref], root);
-  return { head: info.headRefName, base: info.baseRefName };
+  // `gh pr checkout` updates the HEAD but never refreshes the base branch, so a merge-base against
+  // a stale local base makes a long-lived PR show unrelated mainline commits (the reported "PR diff
+  // is wrong"). Refresh the base ref and diff against the remote-tracking tip, matching GitHub's
+  // three-dot "Files changed". Best-effort: offline / a fork base not on `origin` falls back to the
+  // bare branch name (prior behavior). --quiet keeps rev-parse from printing on the miss path.
+  await git(["fetch", "origin", info.baseRefName], root).catch(() => {});
+  const remoteBase = `origin/${info.baseRefName}`;
+  const base = await git(["rev-parse", "--verify", "--quiet", remoteBase], root).then(
+    () => remoteBase,
+    () => info.baseRefName!,
+  );
+  return { head: info.headRefName, base };
 }
 
 // Launch a persistent desk in repo / file / pr mode.

src/git.ts

@@ -6,15 +6,20 @@ import type { DiffFile, DiffHunk, DiffLine } from "./types.js";
 
 const execFileAsync = promisify(execFile);
 
+// Cap on a single git/gh stdout. Generous because a whole-PR `git diff` or a `git show` of a large
+// generated/vendored file can be big; exceeding it rejects (git()) or degrades a file to a spurious
+// full add/delete (fileAt swallows the error), so a too-small cap silently corrupts a large diff.
+const MAX_BUFFER = 256 * 1024 * 1024;
+
 export async function git(args: string[], cwd: string) {
-  const { stdout } = await execFileAsync("git", args, { cwd, maxBuffer: 50 * 1024 * 1024 });
+  const { stdout } = await execFileAsync("git", args, { cwd, maxBuffer: MAX_BUFFER });
   return String(stdout).trimEnd();
 }
 
 // Thin wrapper around the GitHub CLI, used only to resolve a PR number/URL to its branch.
 // Kept optional: callers catch failures (gh missing or unauthenticated) and report them.
 export async function gh(args: string[], cwd: string) {
-  const { stdout } = await execFileAsync("gh", args, { cwd, maxBuffer: 50 * 1024 * 1024 });
+  const { stdout } = await execFileAsync("gh", args, { cwd, maxBuffer: MAX_BUFFER });
   return String(stdout).trimEnd();
 }
 
@@ -126,7 +131,7 @@ export async function fileAt(root: string, rel: string | undefined, ref?: string
     if (ref) {
       const { stdout } = await execFileAsync("git", ["show", `${ref}:${rel}`], {
         cwd: root,
-        maxBuffer: 50 * 1024 * 1024,
+        maxBuffer: MAX_BUFFER,
       });
       return String(stdout);
     }