feat(ui): add fold toggle for unchanged lines between hunks (#303)

Co-authored-by: Ben Vinegar <ben@benv.ca>

Author: adit-chandra

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable user-visible changes to Hunk are documented in this file.
 ### Added
 
 - Added mouse-drag text selection in diff views that copies selected rows to the system clipboard via OSC 52. A `View > Copy decorations` toggle (or `copy_decorations` config) controls whether the clipboard includes diff rails, gutters, and file headers or only the changed code.
+- Added inline expansion for collapsed unchanged file content. Click an unchanged-context row (`▾ N unchanged lines` when expandable, otherwise the static `··· N unchanged lines ···` form) or press `e` while a hunk is selected to reveal surrounding and trailing file lines without leaving the review. The affordance is shown only for input modes that have reachable source content (`hunk diff`, `show`, `stash show`, file-pair `diff` and `difftool`, untracked files); raw `hunk patch` input still renders as before. Failed and in-flight loads surface a one-line status ("Loading…", "Could not load N unchanged lines") on the gap row. Expanded context rows use the same syntax highlighting as the surrounding diff.
 
 ### Changed
 

src/core/diffFile.ts

@@ -2,6 +2,7 @@ import { getFiletypeFromFileName, type FileDiffMetadata } from "@pierre/diffs";
 import { findAgentFileContext } from "./agent";
 import { patchLooksBinary } from "./binary";
 import { normalizeDiffMetadataPaths, normalizeDiffPath } from "./diffPaths";
+import type { FileSourceFetcher } from "./fileSource";
 import type { AgentContext, DiffFile } from "./types";
 
 /** Count visible additions and deletions from parsed diff metadata. */
@@ -21,10 +22,19 @@ export function countDiffStats(metadata: FileDiffMetadata) {
   return { additions, deletions };
 }
 
+export interface DiffFileSourceContext {
+  path: string;
+  previousPath?: string;
+  type: FileDiffMetadata["type"];
+  isUntracked: boolean;
+  isBinary: boolean;
+}
+
 export interface BuildDiffFileOptions {
   isUntracked?: boolean;
   previousPath?: string;
   isBinary?: boolean;
+  sourceFetcherBuilder?: (file: DiffFileSourceContext) => FileSourceFetcher | undefined;
   isTooLarge?: boolean;
   stats?: DiffFile["stats"];
   statsTruncated?: boolean;
@@ -41,6 +51,7 @@ export function buildDiffFile(
     isUntracked,
     previousPath,
     isBinary,
+    sourceFetcherBuilder,
     isTooLarge,
     stats,
     statsTruncated,
@@ -49,6 +60,14 @@ export function buildDiffFile(
   const normalizedMetadata = normalizeDiffMetadataPaths(metadata);
   const path = normalizedMetadata.name;
   const resolvedPreviousPath = normalizeDiffPath(previousPath) ?? normalizedMetadata.prevName;
+  const resolvedIsBinary = isBinary ?? patchLooksBinary(patch);
+  const sourceFetcher = sourceFetcherBuilder?.({
+    path,
+    previousPath: resolvedPreviousPath,
+    type: normalizedMetadata.type,
+    isUntracked: Boolean(isUntracked),
+    isBinary: resolvedIsBinary,
+  });
 
   return {
     id: `${sourcePrefix}:${index}:${path}`,
@@ -60,9 +79,10 @@ export function buildDiffFile(
     metadata: normalizedMetadata,
     agent: findAgentFileContext(agentContext, path, resolvedPreviousPath),
     isUntracked,
-    isBinary: isBinary ?? patchLooksBinary(patch),
+    isBinary: resolvedIsBinary,
     isTooLarge,
     statsTruncated,
+    sourceFetcher,
   };
 }
 

src/core/fileSource.test.ts

@@ -0,0 +1,235 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { createFileSourceFetcher } from "./fileSource";
+
+const tempDirs: string[] = [];
+
+function createTempDir(prefix: string) {
+  const dir = mkdtempSync(join(tmpdir(), prefix));
+  tempDirs.push(dir);
+  return dir;
+}
+
+function git(cwd: string, ...cmd: string[]) {
+  const proc = Bun.spawnSync(["git", ...cmd], {
+    cwd,
+    stdout: "pipe",
+    stderr: "pipe",
+    stdin: "ignore",
+  });
+
+  if (proc.exitCode !== 0) {
+    const stderr = Buffer.from(proc.stderr).toString("utf8");
+    throw new Error(stderr.trim() || `git ${cmd.join(" ")} failed`);
+  }
+
+  return Buffer.from(proc.stdout).toString("utf8");
+}
+
+function createTempRepo(prefix: string) {
+  const dir = createTempDir(prefix);
+  git(dir, "init");
+  git(dir, "config", "user.name", "Test User");
+  git(dir, "config", "user.email", "test@example.com");
+  git(dir, "config", "commit.gpgSign", "false");
+  return dir;
+}
+
+/** Capture console.error calls while exercising diagnostic paths. */
+async function captureConsoleErrors(fn: () => Promise<void>) {
+  const originalConsoleError = console.error;
+  const loggedErrors: unknown[][] = [];
+  console.error = (...args: unknown[]) => {
+    loggedErrors.push(args);
+  };
+
+  try {
+    await fn();
+  } finally {
+    console.error = originalConsoleError;
+  }
+
+  return loggedErrors;
+}
+
+afterEach(() => {
+  while (tempDirs.length > 0) {
+    const dir = tempDirs.pop();
+    if (dir) {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  }
+});
+
+describe("createFileSourceFetcher", () => {
+  test("reads fs paths for old and new sides", async () => {
+    const dir = createTempDir("hunk-source-fs-");
+    const left = join(dir, "before.txt");
+    const right = join(dir, "after.txt");
+    writeFileSync(left, "old contents\n");
+    writeFileSync(right, "new contents\n");
+
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "fs", absolutePath: left },
+      new: { kind: "fs", absolutePath: right },
+    });
+
+    expect(await fetcher.getFullText("old")).toBe("old contents\n");
+    expect(await fetcher.getFullText("new")).toBe("new contents\n");
+  });
+
+  test("returns null for `none` specs", async () => {
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "none" },
+      new: { kind: "none" },
+    });
+
+    expect(await fetcher.getFullText("old")).toBeNull();
+    expect(await fetcher.getFullText("new")).toBeNull();
+  });
+
+  test("returns null when an fs path cannot be read", async () => {
+    const dir = createTempDir("hunk-source-fs-missing-");
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "fs", absolutePath: join(dir, "missing.txt") },
+      new: { kind: "none" },
+    });
+
+    expect(await fetcher.getFullText("old")).toBeNull();
+  });
+
+  test("reads git blob contents for both sides via `git show`", async () => {
+    const repoRoot = createTempRepo("hunk-source-git-");
+    const filePath = "note.txt";
+
+    writeFileSync(join(repoRoot, filePath), "first revision\n");
+    git(repoRoot, "add", ".");
+    git(repoRoot, "commit", "-m", "first");
+    writeFileSync(join(repoRoot, filePath), "second revision\n");
+    git(repoRoot, "add", ".");
+    git(repoRoot, "commit", "-m", "second");
+
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "git-blob", repoRoot, ref: "HEAD~1", path: filePath },
+      new: { kind: "git-blob", repoRoot, ref: "HEAD", path: filePath },
+    });
+
+    expect(await fetcher.getFullText("old")).toBe("first revision\n");
+    expect(await fetcher.getFullText("new")).toBe("second revision\n");
+  });
+
+  test("reads git index contents through an explicit index spec", async () => {
+    const repoRoot = createTempRepo("hunk-source-git-index-");
+    const filePath = "note.txt";
+
+    writeFileSync(join(repoRoot, filePath), "committed\n");
+    git(repoRoot, "add", ".");
+    git(repoRoot, "commit", "-m", "first");
+    writeFileSync(join(repoRoot, filePath), "staged\n");
+    git(repoRoot, "add", filePath);
+    writeFileSync(join(repoRoot, filePath), "working tree\n");
+
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "git-index", repoRoot, path: filePath },
+      new: { kind: "fs", absolutePath: join(repoRoot, filePath) },
+    });
+
+    expect(await fetcher.getFullText("old")).toBe("staged\n");
+    expect(await fetcher.getFullText("new")).toBe("working tree\n");
+  });
+
+  test("passes custom git executable through async git source reads", async () => {
+    const originalSpawn = Bun.spawn;
+    const mutableBun = Bun as unknown as { spawn: typeof Bun.spawn };
+    const spawnCalls: string[][] = [];
+
+    mutableBun.spawn = ((cmds: string[]) => {
+      spawnCalls.push(cmds);
+      return originalSpawn(
+        [
+          process.execPath,
+          "--eval",
+          `process.stdout.write(${JSON.stringify(`read:${cmds[2]}\n`)})`,
+        ],
+        {
+          stdin: "ignore",
+          stdout: "pipe",
+          stderr: "pipe",
+        },
+      );
+    }) as typeof Bun.spawn;
+
+    try {
+      const fetcher = createFileSourceFetcher(
+        {
+          old: { kind: "git-blob", repoRoot: process.cwd(), ref: "HEAD", path: "note.txt" },
+          new: { kind: "git-index", repoRoot: process.cwd(), path: "note.txt" },
+        },
+        { gitExecutable: "custom-git" },
+      );
+
+      expect(await fetcher.getFullText("old")).toBe("read:HEAD:note.txt\n");
+      expect(await fetcher.getFullText("new")).toBe("read::note.txt\n");
+    } finally {
+      mutableBun.spawn = originalSpawn;
+    }
+
+    expect(spawnCalls).toEqual([
+      ["custom-git", "show", "HEAD:note.txt"],
+      ["custom-git", "show", ":note.txt"],
+    ]);
+  });
+
+  test("returns null when a git blob cannot be resolved", async () => {
+    const repoRoot = createTempRepo("hunk-source-git-missing-");
+    writeFileSync(join(repoRoot, "tracked.txt"), "x\n");
+    git(repoRoot, "add", ".");
+    git(repoRoot, "commit", "-m", "first");
+
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "git-blob", repoRoot, ref: "HEAD", path: "missing-from-history.txt" },
+      new: { kind: "none" },
+    });
+
+    const loggedErrors = await captureConsoleErrors(async () => {
+      expect(await fetcher.getFullText("old")).toBeNull();
+    });
+    expect(loggedErrors).toHaveLength(0);
+  });
+
+  test("logs unexpected git source failures with object context", async () => {
+    const repoRoot = createTempDir("hunk-source-git-not-repo-");
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "git-blob", repoRoot, ref: "HEAD", path: "note.txt" },
+      new: { kind: "none" },
+    });
+
+    const loggedErrors = await captureConsoleErrors(async () => {
+      expect(await fetcher.getFullText("old")).toBeNull();
+    });
+
+    expect(loggedErrors).toHaveLength(1);
+    expect(String(loggedErrors[0]?.[0])).toContain("HEAD:note.txt");
+    expect(String(loggedErrors[0]?.[0])).toContain(repoRoot);
+  });
+
+  test("caches resolved text per side", async () => {
+    const dir = createTempDir("hunk-source-cache-");
+    const target = join(dir, "value.txt");
+    writeFileSync(target, "first\n");
+
+    const fetcher = createFileSourceFetcher({
+      old: { kind: "none" },
+      new: { kind: "fs", absolutePath: target },
+    });
+
+    const initial = await fetcher.getFullText("new");
+    writeFileSync(target, "rewritten\n");
+    const cached = await fetcher.getFullText("new");
+
+    expect(initial).toBe("first\n");
+    expect(cached).toBe("first\n");
+  });
+});