Add pop-up window showing the keyboard shortcuts

src/front-end/css/chrome.css

@@ -656,3 +656,46 @@ html:not(.sidebar-resizing) .sidebar {
     margin-inline-start: -14px;
     width: 14px;
 }
+
+/* The container for the help popup that covers the whole window. */
+#mdbook-help-container {
+    /* Position and size for the whole window. */
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    /* This uses flex layout (which is set in book.js), and centers the popup
+       in the window.*/
+    display: none;
+    align-items: center;
+    justify-content: center;
+    z-index: 1000;
+    /* Dim out the book while the popup is visible. */
+    background: var(--overlay-bg);
+}
+
+/* The popup help box. */
+#mdbook-help-popup {
+    box-shadow: 0 4px 24px rgba(0,0,0,0.15);
+    min-width: 300px;
+    max-width: 500px;
+    width: 100%;
+    box-sizing: border-box;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    background-color: var(--bg);
+    color: var(--fg);
+    border-width: 1px;
+    border-color: var(--theme-popup-border);
+    border-style: solid;
+    border-radius: 8px;
+    padding: 10px;
+}
+
+.mdbook-help-title {
+    text-align: center;
+    /* mdbook's margin for h2 is way too large. */
+    margin: 10px;
+}

src/front-end/css/variables.css

@@ -64,6 +64,8 @@
     --copy-button-filter-hover: invert(68%) sepia(55%) saturate(531%) hue-rotate(341deg) brightness(104%) contrast(101%);
 
     --footnote-highlight: #2668a6;
+
+    --overlay-bg: rgba(33, 40, 48, 0.4);
 }
 
 .coal {
@@ -115,6 +117,8 @@
     --copy-button-filter-hover: invert(36%) sepia(70%) saturate(503%) hue-rotate(167deg) brightness(98%) contrast(89%);
 
     --footnote-highlight: #4079ae;
+
+    --overlay-bg: rgba(33, 40, 48, 0.4);
 }
 
 .light, html:not(.js) {
@@ -166,6 +170,8 @@
     --copy-button-filter-hover: invert(14%) sepia(93%) saturate(4250%) hue-rotate(243deg) brightness(99%) contrast(130%);
 
     --footnote-highlight: #7e7eff;
+
+    --overlay-bg: rgba(200, 200, 205, 0.4);
 }
 
 .navy {
@@ -217,6 +223,8 @@
     --copy-button-filter-hover: invert(46%) sepia(20%) saturate(1537%) hue-rotate(156deg) brightness(85%) contrast(90%);
 
     --footnote-highlight: #4079ae;
+
+    --overlay-bg: rgba(33, 40, 48, 0.4);
 }
 
 .rust {
@@ -266,6 +274,8 @@
     --copy-button-filter-hover: invert(77%) sepia(16%) saturate(1798%) hue-rotate(328deg) brightness(98%) contrast(83%);
 
     --footnote-highlight: #d3a17a;
+
+    --overlay-bg: rgba(150, 150, 150, 0.25);
 }
 
 @media (prefers-color-scheme: dark) {

src/front-end/js/book.js

@@ -623,7 +623,7 @@ aria-label="Show hidden lines"></button>';
 
 (function chapterNavigation() {
     document.addEventListener('keydown', function(e) {
-        if (e.altKey || e.ctrlKey || e.metaKey || e.shiftKey) {
+        if (e.altKey || e.ctrlKey || e.metaKey) {
             return;
         }
         if (window.search && window.search.hasFocus()) {
@@ -643,6 +643,55 @@ aria-label="Show hidden lines"></button>';
                 window.location.href = previousButton.href;
             }
         }
+        function showHelp() {
+            const container = document.getElementById('mdbook-help-container');
+            const overlay = document.getElementById('mdbook-help-popup');
+            container.style.display = 'flex';
+
+            // Clicking outside the popup will dismiss it.
+            const mouseHandler = event => {
+                if (overlay.contains(event.target)) {
+                    return;
+                }
+                if (event.button !== 0) {
+                    return;
+                }
+                event.preventDefault();
+                event.stopPropagation();
+                document.removeEventListener('mousedown', mouseHandler);
+                hideHelp();
+            };
+
+            // Pressing esc will dismiss the popup.
+            const escapeKeyHandler = event => {
+                if (event.key === 'Escape') {
+                    event.preventDefault();
+                    event.stopPropagation();
+                    document.removeEventListener('keydown', escapeKeyHandler, true);
+                    hideHelp();
+                }
+            };
+            document.addEventListener('keydown', escapeKeyHandler, true);
+            document.getElementById('mdbook-help-container')
+                .addEventListener('mousedown', mouseHandler);
+        }
+        function hideHelp() {
+            document.getElementById('mdbook-help-container').style.display = 'none';
+        }
+
+        // Usually needs the Shift key to be pressed
+        switch (e.key) {
+        case '?':
+            e.preventDefault();
+            showHelp();
+            break;
+        }
+
+        // Rest of the keys are only active when the Shift key is not pressed
+        if (e.shiftKey) {
+            return;
+        }
+
         switch (e.key) {
         case 'ArrowRight':
             e.preventDefault();

src/front-end/searcher/searcher.js

@@ -33,13 +33,7 @@ window.search = window.search || {};
         mark_exclude = ['text'],
         marker = new Mark(content),
         URL_SEARCH_PARAM = 'search',
-        URL_MARK_PARAM = 'highlight',
-
-        SEARCH_HOTKEY_KEYCODES = [83, 191], // `s` or `/`.
-        ESCAPE_KEYCODE = 27,
-        DOWN_KEYCODE = 40,
-        UP_KEYCODE = 38,
-        SELECT_KEYCODE = 13;
+        URL_MARK_PARAM = 'highlight';
 
     let current_searchterm = '',
         doc_urls = [],
@@ -352,7 +346,7 @@ window.search = window.search || {};
             return;
         }
 
-        if (e.keyCode === ESCAPE_KEYCODE) {
+        if (e.key === 'Escape') {
             e.preventDefault();
             searchbar.classList.remove('active');
             setSearchUrlParameters('',
@@ -362,46 +356,46 @@ window.search = window.search || {};
             }
             showSearch(false);
             marker.unmark();
-        } else if (!hasFocus() && SEARCH_HOTKEY_KEYCODES.includes(e.keyCode)) {
+        } else if (!hasFocus() && (e.key === 'S' || e.key === '/')) {
             e.preventDefault();
             showSearch(true);
             window.scrollTo(0, 0);
             searchbar.select();
-        } else if (hasFocus() && (e.keyCode === DOWN_KEYCODE
-                               || e.keyCode === SELECT_KEYCODE)) {
+        } else if (hasFocus() && (e.key === 'ArrowDown'
+                               || e.key === 'Enter')) {
             e.preventDefault();
             const first = searchresults.firstElementChild;
             if (first !== null) {
                 unfocusSearchbar();
                 first.classList.add('focus');
-                if (e.keyCode === SELECT_KEYCODE) {
+                if (e.key === 'Enter') {
                     window.location.assign(first.querySelector('a'));
                 }
             }
-        } else if (!hasFocus() && (e.keyCode === DOWN_KEYCODE
-                                || e.keyCode === UP_KEYCODE
-                                || e.keyCode === SELECT_KEYCODE)) {
+        } else if (!hasFocus() && (e.key === 'ArrowDown'
+                                || e.key === 'ArrowUp'
+                                || e.key === 'Enter')) {
             // not `:focus` because browser does annoying scrolling
             const focused = searchresults.querySelector('li.focus');
             if (!focused) {
                 return;
             }
             e.preventDefault();
-            if (e.keyCode === DOWN_KEYCODE) {
+            if (e.key === 'ArrowDown') {
                 const next = focused.nextElementSibling;
                 if (next) {
                     focused.classList.remove('focus');
                     next.classList.add('focus');
                 }
-            } else if (e.keyCode === UP_KEYCODE) {
+            } else if (e.key === 'ArrowUp') {
                 focused.classList.remove('focus');
                 const prev = focused.previousElementSibling;
                 if (prev) {
                     prev.classList.add('focus');
                 } else {
                     searchbar.select();
                 }
-            } else { // SELECT_KEYCODE
+            } else { // Enter
                 window.location.assign(focused.querySelector('a'));
             }
         }

src/front-end/templates/index.hbs

@@ -63,6 +63,19 @@
         <script src="{{ resource "toc.js" }}"></script>
     </head>
     <body>
+    <div id="mdbook-help-container">
+        <div id="mdbook-help-popup">
+            <h2 class="mdbook-help-title">Keyboard shortcuts</h2>
+            <div>
+                <p>Press <kbd>←</kbd> or <kbd>→</kbd> to navigate between chapters</p>
+                {{#if search_enabled}}
+                <p>Press <kbd>S</kbd> or <kbd>/</kbd> to search in the book</p>
+                {{/if}}
+                <p>Press <kbd>?</kbd> to show this help</p>
+                <p>Press <kbd>Esc</kbd> to hide this help</p>
+            </div>
+        </div>
+    </div>
     <div id="body-container">
         <!-- Work around some values being stored in localStorage wrapped in quotes -->
         <script>

src/renderer/html_handlebars/hbs_renderer.rs

@@ -667,7 +667,7 @@ fn build_header_links(html: &str) -> String {
     static BUILD_HEADER_LINKS: LazyLock<Regex> = LazyLock::new(|| {
         Regex::new(r#"<h(\d)(?: id="([^"]+)")?(?: class="([^"]+)")?>(.*?)</h\d>"#).unwrap()
     });
-    static IGNORE_CLASS: &[&str] = &["menu-title"];
+    static IGNORE_CLASS: &[&str] = &["menu-title", "mdbook-help-title"];
 
     let mut id_counter = HashMap::new();