Fix preview browse URL including output filename for single files

PR #13804 made project always non-null via singleFileProjectContext(),
causing the initialPath computation to always take the project branch.
This produced URLs like http://localhost:PORT/hello.html instead of
http://localhost:PORT/ for standalone files, breaking Posit Workbench
proxy access.

Guard with project.isSingleFile so single-file previews use root path.
Extract computation into previewInitialPath() for testability.

Fixes #14298

Author: cderv

src/command/preview/preview.ts

@@ -143,6 +143,21 @@ interface PreviewOptions {
   presentation: boolean;
 }
 
+export function previewInitialPath(
+  outputFile: string,
+  project: ProjectContext | undefined,
+): string {
+  if (isPdfContent(outputFile)) {
+    return kPdfJsInitialPath;
+  }
+  if (project && !project.isSingleFile) {
+    return pathWithForwardSlashes(
+      relative(projectOutputDir(project), outputFile),
+    );
+  }
+  return "";
+}
+
 export async function preview(
   file: string,
   flags: RenderFlags,
@@ -253,13 +268,7 @@ export async function preview(
     );
 
   // open browser if this is a browseable format
-  const initialPath = isPdfContent(result.outputFile)
-    ? kPdfJsInitialPath
-    : project
-    ? pathWithForwardSlashes(
-      relative(projectOutputDir(project), result.outputFile),
-    )
-    : "";
+  const initialPath = previewInitialPath(result.outputFile, project);
   if (
     options.browser &&
     !isServerSession() &&

tests/unit/preview-initial-path.test.ts

@@ -0,0 +1,73 @@
+/*
+ * preview-initial-path.test.ts
+ *
+ * Tests that previewInitialPath computes the correct browse URL path
+ * for different project types. Regression test for #14298.
+ *
+ * Copyright (C) 2026 Posit Software, PBC
+ */
+
+import { unitTest } from "../test.ts";
+import { assertEquals } from "testing/asserts";
+import { join } from "../../src/deno_ral/path.ts";
+import { normalizePath } from "../../src/core/path.ts";
+import { previewInitialPath } from "../../src/command/preview/preview.ts";
+import { ProjectContext } from "../../src/project/types.ts";
+
+function mockProjectContext(
+  dir: string,
+  isSingleFile: boolean,
+): ProjectContext {
+  return {
+    dir: normalizePath(dir),
+    isSingleFile,
+    engines: [],
+    files: { input: [], resources: [], config: [], configResources: [] },
+    config: { project: {} },
+    notebookContext: () => ({ resolve: () => undefined, get: () => undefined }),
+    resolveFullMarkdownForFile: () => Promise.resolve(undefined),
+    cleanup: () => {},
+  } as unknown as ProjectContext;
+}
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - single file returns empty path (#14298)", async () => {
+  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
+  const outputFile = join(dir, "hello.html");
+  const project = mockProjectContext(dir, true);
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "", "Single-file preview should use root path, not filename");
+
+  Deno.removeSync(dir, { recursive: true });
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - project file returns relative path", async () => {
+  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
+  const outputFile = join(dir, "chapter.html");
+  const project = mockProjectContext(dir, false);
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "chapter.html", "Project preview should include relative path");
+
+  Deno.removeSync(dir, { recursive: true });
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - project subdir returns relative path", async () => {
+  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
+  const outputFile = join(dir, "pages", "about.html");
+  const project = mockProjectContext(dir, false);
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "pages/about.html", "Project preview should include subdirectory path");
+
+  Deno.removeSync(dir, { recursive: true });
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - undefined project returns empty path", async () => {
+  const result = previewInitialPath("/tmp/hello.html", undefined);
+  assertEquals(result, "", "No project should use root path");
+});