Make the sidebar work without JS

Uses an iframe instead. The downside of iframes comes from them
not necessarily being same-origin as the main page (particularly
with `file:///` URLs), which can cause themes to fall out of sync,
but that's not a problem here since themes don't work without JS
anyway.

src/renderer/html_handlebars/hbs_renderer.rs

@@ -529,7 +529,9 @@ impl Renderer for HtmlHandlebars {
         handlebars.register_partial("header", String::from_utf8(theme.header.clone())?)?;
 
         debug!("Register the toc handlebars template");
-        handlebars.register_template_string("toc", String::from_utf8(theme.toc.clone())?)?;
+        handlebars.register_template_string("toc_js", String::from_utf8(theme.toc_js.clone())?)?;
+        handlebars
+            .register_template_string("toc_html", String::from_utf8(theme.toc_html.clone())?)?;
 
         debug!("Register handlebars helpers");
         self.register_hbs_helpers(&mut handlebars, &html_config);
@@ -586,11 +588,16 @@ impl Renderer for HtmlHandlebars {
             debug!("Creating print.html ✓");
         }
 
-        debug!("Render toc.js");
+        debug!("Render toc");
         {
-            let rendered_toc = handlebars.render("toc", &data)?;
+            let rendered_toc = handlebars.render("toc_js", &data)?;
             utils::fs::write_file(destination, "toc.js", rendered_toc.as_bytes())?;
             debug!("Creating toc.js ✓");
+            data.insert("is_toc_html".to_owned(), json!(true));
+            let rendered_toc = handlebars.render("toc_html", &data)?;
+            utils::fs::write_file(destination, "toc.html", rendered_toc.as_bytes())?;
+            debug!("Creating toc.html ✓");
+            data.remove("is_toc_html");
         }
 
         debug!("Copy static files");

src/renderer/html_handlebars/helpers/toc.rs

@@ -48,6 +48,13 @@ impl HelperDef for RenderToc {
                 RenderErrorReason::Other("Type error for `fold_level`, u64 expected".to_owned())
             })?;
 
+        // If true, then this is the iframe and we need target="_parent"
+        let is_toc_html = rc
+            .evaluate(ctx, "@root/is_toc_html")?
+            .as_json()
+            .as_bool()
+            .unwrap_or(false);
+
         out.write("<ol class=\"chapter\">")?;
 
         let mut current_level = 1;
@@ -113,7 +120,11 @@ impl HelperDef for RenderToc {
 
                     // Add link
                     out.write(&tmp)?;
-                    out.write("\">")?;
+                    out.write(if is_toc_html {
+                        "\" target=\"_parent\">"
+                    } else {
+                        "\">"
+                    })?;
                     path_exists = true;
                 }
                 _ => {

src/theme/css/chrome.css

@@ -399,6 +399,22 @@ ul#searchresults span.teaser em {
     background-color: var(--sidebar-bg);
     color: var(--sidebar-fg);
 }
+.sidebar-iframe-inner {
+    background-color: var(--sidebar-bg);
+    color: var(--sidebar-fg);
+    padding: 10px 10px;
+    margin: 0;
+    font-size: 1.4rem;
+}
+.sidebar-iframe-outer {
+    border: none;
+    height: 100%;
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+}
 [dir=rtl] .sidebar { left: unset; right: 0; }
 .sidebar-resizing {
     -moz-user-select: none;

src/theme/index.hbs

@@ -111,6 +111,9 @@
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
             <!-- populated by js -->
             <div class="sidebar-scrollbox"></div>
+            <noscript>
+                <iframe class="sidebar-iframe-outer" src="{{ path_to_root }}toc.html"></iframe>
+            </noscript>
             <div id="sidebar-resize-handle" class="sidebar-resize-handle">
                 <div class="sidebar-resize-indicator"></div>
             </div>

src/theme/mod.rs

@@ -17,7 +17,8 @@ pub static INDEX: &[u8] = include_bytes!("index.hbs");
 pub static HEAD: &[u8] = include_bytes!("head.hbs");
 pub static REDIRECT: &[u8] = include_bytes!("redirect.hbs");
 pub static HEADER: &[u8] = include_bytes!("header.hbs");
-pub static TOC: &[u8] = include_bytes!("toc.js.hbs");
+pub static TOC_JS: &[u8] = include_bytes!("toc.js.hbs");
+pub static TOC_HTML: &[u8] = include_bytes!("toc.html.hbs");
 pub static CHROME_CSS: &[u8] = include_bytes!("css/chrome.css");
 pub static GENERAL_CSS: &[u8] = include_bytes!("css/general.css");
 pub static PRINT_CSS: &[u8] = include_bytes!("css/print.css");
@@ -51,7 +52,8 @@ pub struct Theme {
     pub head: Vec<u8>,
     pub redirect: Vec<u8>,
     pub header: Vec<u8>,
-    pub toc: Vec<u8>,
+    pub toc_js: Vec<u8>,
+    pub toc_html: Vec<u8>,
     pub chrome_css: Vec<u8>,
     pub general_css: Vec<u8>,
     pub print_css: Vec<u8>,
@@ -87,7 +89,8 @@ impl Theme {
                 (theme_dir.join("head.hbs"), &mut theme.head),
                 (theme_dir.join("redirect.hbs"), &mut theme.redirect),
                 (theme_dir.join("header.hbs"), &mut theme.header),
-                (theme_dir.join("toc.js.hbs"), &mut theme.toc),
+                (theme_dir.join("toc.js.hbs"), &mut theme.toc_js),
+                (theme_dir.join("toc.html.hbs"), &mut theme.toc_html),
                 (theme_dir.join("book.js"), &mut theme.js),
                 (theme_dir.join("css/chrome.css"), &mut theme.chrome_css),
                 (theme_dir.join("css/general.css"), &mut theme.general_css),
@@ -177,7 +180,8 @@ impl Default for Theme {
             head: HEAD.to_owned(),
             redirect: REDIRECT.to_owned(),
             header: HEADER.to_owned(),
-            toc: TOC.to_owned(),
+            toc_js: TOC_JS.to_owned(),
+            toc_html: TOC_HTML.to_owned(),
             chrome_css: CHROME_CSS.to_owned(),
             general_css: GENERAL_CSS.to_owned(),
             print_css: PRINT_CSS.to_owned(),
@@ -237,6 +241,7 @@ mod tests {
             "redirect.hbs",
             "header.hbs",
             "toc.js.hbs",
+            "toc.html.hbs",
             "favicon.png",
             "favicon.svg",
             "css/chrome.css",
@@ -268,7 +273,8 @@ mod tests {
             head: Vec::new(),
             redirect: Vec::new(),
             header: Vec::new(),
-            toc: Vec::new(),
+            toc_js: Vec::new(),
+            toc_html: Vec::new(),
             chrome_css: Vec::new(),
             general_css: Vec::new(),
             print_css: Vec::new(),

src/theme/toc.html.hbs

@@ -0,0 +1,43 @@
+<!DOCTYPE HTML>
+<html lang="{{ language }}" class="{{ default_theme }}" dir="{{ text_direction }}">
+    <head>
+        <!-- sidebar iframe generated using mdBook
+
+        This is a frame, and not included directly in the page, to control the total size of the
+        book. The TOC contains an entry for each page, so if each page includes a copy of the TOC,
+        the total size of the page becomes O(n**2).
+
+        The frame is only used as a fallback when JS is turned off. When it's on, the sidebar is
+        instead added to the main page by `toc.js` instead. The JavaScript mode is better
+        because, when running in a `file:///` URL, the iframed page would not be Same-Origin as
+        the rest of the page, so the sidebar and the main page theme would fall out of sync.
+        -->
+        <meta charset="UTF-8">
+        <meta name="robots" content="noindex">
+        {{#if base_url}}
+        <base href="{{ base_url }}">
+        {{/if}}
+        <!-- Custom HTML head -->
+        {{> head}}
+        <meta name="viewport" content="width=device-width, initial-scale=1">
+        <meta name="theme-color" content="#ffffff">
+        <link rel="stylesheet" href="{{ path_to_root }}css/variables.css">
+        <link rel="stylesheet" href="{{ path_to_root }}css/general.css">
+        <link rel="stylesheet" href="{{ path_to_root }}css/chrome.css">
+        {{#if print_enable}}
+        <link rel="stylesheet" href="{{ path_to_root }}css/print.css" media="print">
+        {{/if}}
+        <!-- Fonts -->
+        <link rel="stylesheet" href="{{ path_to_root }}FontAwesome/css/font-awesome.css">
+        {{#if copy_fonts}}
+        <link rel="stylesheet" href="{{ path_to_root }}fonts/fonts.css">
+        {{/if}}
+        <!-- Custom theme stylesheets -->
+        {{#each additional_css}}
+        <link rel="stylesheet" href="{{ ../path_to_root }}{{ this }}">
+        {{/each}}
+    </head>
+    <body class="sidebar-iframe-inner">
+        {{#toc}}{{/toc}}
+    </body>
+</html>

tests/rendered_output.rs

@@ -9,7 +9,7 @@ use mdbook::utils::fs::write_file;
 use mdbook::MDBook;
 use pretty_assertions::assert_eq;
 use select::document::Document;
-use select::predicate::{Class, Name, Predicate};
+use select::predicate::{Attr, Class, Name, Predicate};
 use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::fs;
@@ -232,7 +232,7 @@ fn entry_ends_with(entry: &DirEntry, ending: &str) -> bool {
 
 /// Read the TOC (`book/toc.js`) nested HTML and expose it as a DOM which we
 /// can search with the `select` crate
-fn toc_html() -> Result<Document> {
+fn toc_js_html() -> Result<Document> {
     let temp = DummyBook::new()
         .build()
         .with_context(|| "Couldn't create the dummy book")?;
@@ -252,9 +252,24 @@ fn toc_html() -> Result<Document> {
     panic!("cannot find toc in file")
 }
 
+/// Read the TOC fallback (`book/toc.html`) HTML and expose it as a DOM which we
+/// can search with the `select` crate
+fn toc_fallback_html() -> Result<Document> {
+    let temp = DummyBook::new()
+        .build()
+        .with_context(|| "Couldn't create the dummy book")?;
+    MDBook::load(temp.path())?
+        .build()
+        .with_context(|| "Book building failed")?;
+
+    let toc_path = temp.path().join("book").join("toc.html");
+    let html = fs::read_to_string(toc_path).with_context(|| "Unable to read index.html")?;
+    Ok(Document::from(html.as_str()))
+}
+
 #[test]
 fn check_second_toc_level() {
-    let doc = toc_html().unwrap();
+    let doc = toc_js_html().unwrap();
     let mut should_be = Vec::from(TOC_SECOND_LEVEL);
     should_be.sort_unstable();
 
@@ -276,7 +291,7 @@ fn check_second_toc_level() {
 
 #[test]
 fn check_first_toc_level() {
-    let doc = toc_html().unwrap();
+    let doc = toc_js_html().unwrap();
     let mut should_be = Vec::from(TOC_TOP_LEVEL);
 
     should_be.extend(TOC_SECOND_LEVEL);
@@ -299,7 +314,7 @@ fn check_first_toc_level() {
 
 #[test]
 fn check_spacers() {
-    let doc = toc_html().unwrap();
+    let doc = toc_js_html().unwrap();
     let should_be = 2;
 
     let num_spacers = doc
@@ -308,6 +323,39 @@ fn check_spacers() {
     assert_eq!(num_spacers, should_be);
 }
 
+// don't use target="_parent" in JS
+#[test]
+fn check_link_target_js() {
+    let doc = toc_js_html().unwrap();
+
+    let num_parent_links = doc
+        .find(
+            Class("chapter")
+                .descendant(Name("li"))
+                .descendant(Name("a").and(Attr("target", "_parent"))),
+        )
+        .count();
+    assert_eq!(num_parent_links, 0);
+}
+
+// don't use target="_parent" in IFRAME
+#[test]
+fn check_link_target_fallback() {
+    let doc = toc_fallback_html().unwrap();
+
+    let num_parent_links = doc
+        .find(
+            Class("chapter")
+                .descendant(Name("li"))
+                .descendant(Name("a").and(Attr("target", "_parent"))),
+        )
+        .count();
+    assert_eq!(
+        num_parent_links,
+        TOC_TOP_LEVEL.len() + TOC_SECOND_LEVEL.len()
+    );
+}
+
 /// Ensure building fails if `create-missing` is false and one of the files does
 /// not exist.
 #[test]