diff --git a/doc/doc-support/package.nix b/doc/doc-support/package.nix
index 466beb204425..f2c37df39132 100644
--- a/doc/doc-support/package.nix
+++ b/doc/doc-support/package.nix
@@ -73,6 +73,7 @@ stdenvNoCC.mkDerivation (
             ../anchor.min.js
             ../manpage-urls.json
             ../redirects.json
+            ../nav.json
           ]
         );
     };
@@ -117,6 +118,7 @@ stdenvNoCC.mkDerivation (
         --script ./anchor.min.js \
         --script ./anchor-use.js \
         --sidebar-depth 3 \
+        --nav ./nav.json \
         manual.md \
         out/index.html
 
diff --git a/doc/nav.json b/doc/nav.json
new file mode 100644
index 000000000000..15d4926421cf
--- /dev/null
+++ b/doc/nav.json
@@ -0,0 +1,3 @@
+{
+  "open": []
+}
diff --git a/doc/style.css b/doc/style.css
index 90517caf4948..ac052c81511c 100644
--- a/doc/style.css
+++ b/doc/style.css
@@ -131,6 +131,32 @@ body {
     padding-left: 1rem;
     padding-right: 1rem;
   }
+
+  /*
+  See: https://developer.mozilla.org/en-US/docs/Web/API/Popover_API
+
+  - :popover-open pseudo-class matches a popover element when it is in the showing state
+  - ::backdrop full-screen element placed directly behind popover
+  */
+  nav.toc-sidebar:popover-open {
+    position: fixed;
+    inset: 0 auto 0 0;
+    width: min(20rem, 85vw);
+    height: 100dvh;
+    max-height: none;
+    margin: 0;
+    padding: 1rem;
+    overflow-y: auto;
+    overscroll-behavior: contain;
+    background: var(--background);
+    border: 0;
+    border-right: 0.0625rem solid #d8d8d8;
+    box-shadow: 0 0 1.5rem rgb(0 0 0 / 0.35);
+  }
+
+  nav.toc-sidebar::backdrop {
+    background: rgb(0 0 0 / 0.5);
+  }
 }
 
 a {
@@ -486,9 +512,25 @@ div.appendix .variablelist .term {
 
 nav.toc-sidebar {
   height: 100%;
-  overflow-y: none;
   padding: 0 1rem 2rem;
-  border-bottom: 0.0625rem solid #d8d8d8;
+}
+
+/* menu button, shown on mobile, hidden on desktop */
+.toc-toggle {
+  display: inline-flex;
+  align-items: center;
+  margin: 0.75rem 0 0;
+  padding: 0.4rem 0.8rem;
+  border: 0.0625rem solid #d8d8d8;
+  border-radius: 0.25rem;
+  background: var(--background);
+  color: var(--main-text-color);
+  font: inherit;
+  cursor: pointer;
+  position: fixed;
+  top: 0.5rem;
+  left: 0.8rem;
+  z-index: 2;
 }
 
 nav.toc-sidebar .toc {
@@ -533,14 +575,25 @@ nav.toc-sidebar summary {
   }
 
   nav.toc-sidebar {
+    /* un-pop the drawer */
+    display: block;
+    position: static;
+    inset: auto;
+    /*  */
+    margin: 0;
     grid-column: 1;
     grid-row: 2;
     max-height: none;
     overflow-y: auto;
-    border-bottom: none;
+    border: none;
     border-right: 0.0625rem solid #d8d8d8;
   }
 
+  /* Hide the toggle button on desktop */
+  .toc-toggle {
+    display: none;
+  }
+
   main.content {
     grid-column: 1 / -1;
     grid-row: 2;
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
index 902b365d8b5f..11d247320d68 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
@@ -264,6 +264,7 @@ class HTMLParameters(NamedTuple):
     # structural depth of the navigation sidebar tree
     sidebar_depth: int
     media_dir: Path
+    sidebar_open: frozenset[str] = frozenset()
 
 class ManualHTMLRenderer(RendererMixin, HTMLRenderer):
     _base_path: Path
@@ -367,6 +368,20 @@ class ManualHTMLRenderer(RendererMixin, HTMLRenderer):
                 file.write(self._redirects.get_redirect_script(toc.target.path))
             scripts.append(f'./{redirects_name}')
 
+        # Register a close handler
+        # Without this the popover can still be closed by clicking outside of it
+        # It handles auto-closing when the user clicks a href.
+        close_menu_js = """
+            document.addEventListener("DOMContentLoaded", () => {
+                const nav = document.getElementById("manual-toc");
+                nav?.addEventListener("click", (e) => {
+                    if (e.target.closest("a[href]") && nav.matches(":popover-open")) {
+                        nav.hidePopover();
+                    }
+                });
+            });
+        """
+
         return "\n".join([
             '<?xml version="1.0" encoding="utf-8" standalone="no"?>',
             '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"',
@@ -380,13 +395,19 @@ class ManualHTMLRenderer(RendererMixin, HTMLRenderer):
                      for style in self._html_params.stylesheets)),
             "".join((f'<script src="{html.escape(script, True)}" type="text/javascript"></script>'
                      for script in scripts)),
+            f"<script>{close_menu_js}</script>",
             f' <meta name="generator" content="{html.escape(self._html_params.generator, True)}" />',
             f' <link rel="home" href="{home.target.href()}" title="{home.target.title}" />' if home.target.href() else "",
             f' {up_link}{prev_link}{next_link}',
             ' </head>',
             ' <body>',
+            # See: https://developer.mozilla.org/en-US/docs/Web/API/Popover_API
+            # Supported by most browsers since 2023, full support since Jan 2025
+            ('  <button type="button" class="toc-toggle" popovertarget="manual-toc"'
+             ' popovertargetaction="toggle" aria-label="Toggle table of contents">'
+             '☰</button>') if sidebar else "",
             nav_html,
-            f'  <nav class="toc-sidebar">{sidebar}</nav>' if sidebar else "",
+            f'  <nav id="manual-toc" class="toc-sidebar" popover="auto">{sidebar}</nav>' if sidebar else "",
             '  <main class="content">',
         ])
 
@@ -450,9 +471,10 @@ class ManualHTMLRenderer(RendererMixin, HTMLRenderer):
                 link = f'<a href="{e.target.href()}">{e.target.toc_html}</a>'
                 cls = html.escape(e.kind, True)
                 if children:
+                    open_attr = " open" if e.target.id in self._html_params.sidebar_open else ""
                     items.append(
                         f'<li class="{cls}">'
-                        f'<details open><summary>{link}</summary>{children}</details>'
+                        f'<details{open_attr}><summary>{link}</summary>{children}</details>'
                         '</li>'
                     )
                 else:
@@ -729,6 +751,9 @@ def _build_cli_html(p: argparse.ArgumentParser) -> None:
     p.add_argument('--media-dir', default="media", type=Path)
     p.add_argument('--redirects', type=Path)
     p.add_argument('--sidebar-depth', default=2, type=int)
+    # nav metadata (JSON): {"open": ["anchor-id", ...]} selects which sidebar
+    # entries render expanded; omitted or absent means everything is collapsed.
+    p.add_argument('--nav', type=Path)
     # Deprecated flags,
     p.add_argument('--toc-depth', nargs='?', action=_DeprecatedDepthFlag, default=None)
     p.add_argument('--chunk-toc-depth', nargs='?', action=_DeprecatedDepthFlag, default=None)
@@ -738,6 +763,10 @@ def _build_cli_html(p: argparse.ArgumentParser) -> None:
     p.add_argument('outfile', type=Path)
 
 def _run_cli_html(args: argparse.Namespace) -> None:
+    sidebar_open: frozenset[str] = frozenset()
+    if args.nav:
+        with open(args.nav) as nav_file:
+            sidebar_open = frozenset(json.load(nav_file).get("open", []))
     with open(args.manpage_urls) as manpage_urls, open(Path(__file__).parent / "redirects.js") as redirects_script:
         redirects = None
         if args.redirects:
@@ -747,7 +776,7 @@ def _run_cli_html(args: argparse.Namespace) -> None:
         md = HTMLConverter(
             args.revision,
             HTMLParameters(args.generator, args.stylesheet, args.script,
-                           args.sidebar_depth, args.media_dir),
+                           args.sidebar_depth, args.media_dir, sidebar_open),
             json.load(manpage_urls), redirects)
         md.convert(args.infile, args.outfile)
 
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
index 339bf58b2462..14f7024ce186 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
@@ -3,7 +3,7 @@ from pathlib import Path
 from nixos_render_docs.manual import HTMLConverter, HTMLParameters
 
 
-def _build(tmp_path: Path, sidebar_depth: int = 2) -> str:
+def _build(tmp_path: Path, sidebar_depth: int = 2, sidebar_open: frozenset[str] = frozenset()) -> str:
     (tmp_path / "part.md").write_text(
         "# Build helpers {#part-builders}\n\n" # -> h2
         "```{=include=} chapters\nchapter.md\n```\n"
@@ -29,7 +29,7 @@ def _build(tmp_path: Path, sidebar_depth: int = 2) -> str:
     out.mkdir(exist_ok=True)
     conv = HTMLConverter(
         "1.0.0",
-        HTMLParameters("test-gen", [], [], sidebar_depth, Path("media")),
+        HTMLParameters("test-gen", [], [], sidebar_depth, Path("media"), sidebar_open),
         {},
     )
     conv.convert(tmp_path / "index.md", out / "index.html")
@@ -53,11 +53,12 @@ def test_single_h1_and_flat_heading_levels(tmp_path: Path) -> None:
 
 def test_sidebar_is_collapsible_tree(tmp_path: Path) -> None:
     html = _build(tmp_path)
-    assert '<nav class="toc-sidebar">' in html
+    assert '<nav id="manual-toc" class="toc-sidebar" popover="auto">' in html
+    assert 'popovertarget="manual-toc"' in html  # the toggle button
     assert '<ol class="toc">' in html
-    # entries with children collapse into <details>, open by default.
-    assert "<details open><summary>" in html
-    assert "<details>" not in html # There is no tag without open
+    # Entries with children collapse into <details>; closed by default.
+    assert "<details><summary>" in html
+    assert "<details open>" not in html  # nothing opens without nav metadata
     # No more inline TOCs, this makes the output visually hard to parse
     assert "Table of Contents" not in html
     # Sidebar links to structural entries
@@ -66,6 +67,13 @@ def test_sidebar_is_collapsible_tree(tmp_path: Path) -> None:
     assert 'href="#sec-first"' in html
 
 
+def test_nav_metadata_opens_selected_entries(tmp_path: Path) -> None:
+    # ids listed in the nav "open" set render as <details open>
+    html = _build(tmp_path, sidebar_depth=3, sidebar_open=frozenset({"chap-fpa"}))
+    assert '<details open><summary><a href="#chap-fpa"' in html
+    assert '<details><summary><a href="#part-builders"' in html
+
+
 def test_sidebar_depth_caps_the_tree(tmp_path: Path) -> None:
     # sub-a is h3 in a .chapter.md
     # with depth=3, it gets listed
@@ -105,7 +113,7 @@ def test_chunked_pages_carry_the_sidebar(tmp_path: Path) -> None:
     )
     conv.convert(tmp_path / "index.md", out / "index.html")
     chunk = (out / "chapter.html").read_text()
-    assert '<nav class="toc-sidebar">' in chunk
+    assert '<nav id="manual-toc" class="toc-sidebar" popover="auto">' in chunk
     assert '<ol class="toc">' in chunk
     assert 'href="chapter.html#sec-first"' in chunk
     # All headings visible in the chunk sidebar