fix: resolve file compare paths against the git root

src/commands/compareFileWithRev.ts

@@ -1,6 +1,7 @@
 import * as vscode from "vscode";
 import { REV_KINDS, TITLE_PREFIX } from "../constants";
 import type { GitRepo } from "../git/GitRepo";
+import { toRepoRelPath } from "../git/relPath";
 import type { Sha } from "../git/types";
 import type { Result } from "../result";
 import { err, ok } from "../result";
@@ -69,6 +70,7 @@ const handler = async ({
     void vscode.window.showWarningMessage(NOT_IN_REPO);
     return;
   }
+  const repoRoot = vsRepo.rootUri.fsPath;
   const repo = buildRepo(deps.runner, vsRepo);
   const sha = await pickShaForFile({ repo, source, output: deps.output });
   if (!sha.ok) {
@@ -77,8 +79,8 @@ const handler = async ({
   await openDiff({
     revA: sideAFromSha(sha.value),
     revB: { kind: REV_KINDS.workingCopy },
-    repoRoot: vsRepo.rootUri.fsPath,
-    relPath: vscode.workspace.asRelativePath(target, false),
+    repoRoot,
+    relPath: toRepoRelPath({ repoRoot, fileFsPath: target.fsPath }),
   });
 };
 

src/git/relPath.ts

@@ -0,0 +1,18 @@
+import * as path from "node:path";
+
+const GIT_PATH_SEP = "/";
+
+/**
+ * File path relative to the git repository root, with forward slashes.
+ *
+ * Must NOT use workspace-folder-relative paths: the VS Code workspace folder
+ * can be a subdirectory of the git root (nested package / multi-root), and
+ * joining that with the git root produces a wrong absolute path.
+ */
+export const toRepoRelPath = ({
+  repoRoot,
+  fileFsPath,
+}: {
+  repoRoot: string;
+  fileFsPath: string;
+}): string => path.relative(repoRoot, fileFsPath).split(path.sep).join(GIT_PATH_SEP);

src/test/unit/relPath.test.ts

@@ -0,0 +1,39 @@
+import { strict as assert } from "node:assert";
+import * as path from "node:path";
+import { toRepoRelPath } from "../../git/relPath";
+
+describe("toRepoRelPath", () => {
+  it("keeps every segment under the git root when the workspace folder is a nested package", () => {
+    const repoRoot = "/repo";
+    const fileFsPath = "/repo/scripts/pkg/pkg/module.py";
+
+    // Reproduce the product bug: asRelativePath(workspace=.../scripts/pkg)
+    // yields "pkg/module.py", then path.join(gitRoot, that) is wrong.
+    const nestedWorkspace = "/repo/scripts/pkg";
+    const workspaceRel = path.relative(nestedWorkspace, fileFsPath);
+    assert.equal(workspaceRel, "pkg/module.py");
+    assert.equal(path.join(repoRoot, workspaceRel), "/repo/pkg/module.py");
+
+    assert.equal(toRepoRelPath({ repoRoot, fileFsPath }), "scripts/pkg/pkg/module.py");
+  });
+
+  it("returns a single-segment path for a file at the repo root", () => {
+    assert.equal(
+      toRepoRelPath({
+        repoRoot: "/repo",
+        fileFsPath: "/repo/README.md",
+      }),
+      "README.md"
+    );
+  });
+
+  it("preserves nested directories under the repo root", () => {
+    assert.equal(
+      toRepoRelPath({
+        repoRoot: "/repo",
+        fileFsPath: "/repo/dir/c.txt",
+      }),
+      "dir/c.txt"
+    );
+  });
+});