Fix asciidoctor callouts with highlight.js 11.x

See:
* highlightjs/highlight.js#2889
* asciidoctor/asciidoctor#3976

layouts/partials/highlight-js.html

@@ -8,6 +8,196 @@
         {{ end }}
     {{ end }}
     <script type="text/javascript">
+        // highlight.js 11 removed the 'HTML auto-merging' internal plugin
+        // however this is required for AsciiDoc to insert callouts
+        // fortunately the issue description shows how to re-enable it, by registering the plugin
+        // Copy-pasted code from https://github.com/highlightjs/highlight.js/issues/2889
+        // See also 
+        var mergeHTMLPlugin = (function () {
+            'use strict';
+
+            var originalStream;
+
+            /**
+             * @param {string} value
+             * @returns {string}
+             */
+            function escapeHTML(value) {
+                return value
+                    .replace(/&/g, '&amp;')
+                    .replace(/</g, '&lt;')
+                    .replace(/>/g, '&gt;')
+                    .replace(/"/g, '&quot;')
+                    .replace(/'/g, '&#x27;');
+            }
+
+            /* plugin itself */
+
+            /** @type {HLJSPlugin} */
+            const mergeHTMLPlugin = {
+                // preserve the original HTML token stream
+                "before:highlightElement": ({ el }) => {
+                    originalStream = nodeStream(el);
+                },
+                // merge it afterwards with the highlighted token stream
+                "after:highlightElement": ({ el, result, text }) => {
+                    if (!originalStream.length) return;
+
+                    const resultNode = document.createElement('div');
+                    resultNode.innerHTML = result.value;
+                    result.value = mergeStreams(originalStream, nodeStream(resultNode), text);
+                    el.innerHTML = result.value;
+                }
+            };
+
+            /* Stream merging support functions */
+
+            /**
+             * @typedef Event
+             * @property {'start'|'stop'} event
+             * @property {number} offset
+             * @property {Node} node
+             */
+
+            /**
+             * @param {Node} node
+             */
+            function tag(node) {
+                return node.nodeName.toLowerCase();
+            }
+
+            /**
+             * @param {Node} node
+             */
+            function nodeStream(node) {
+                /** @type Event[] */
+                const result = [];
+                (function _nodeStream(node, offset) {
+                    for (let child = node.firstChild; child; child = child.nextSibling) {
+                        if (child.nodeType === 3) {
+                            offset += child.nodeValue.length;
+                        } else if (child.nodeType === 1) {
+                            result.push({
+                                event: 'start',
+                                offset: offset,
+                                node: child
+                            });
+                            offset = _nodeStream(child, offset);
+                            // Prevent void elements from having an end tag that would actually
+                            // double them in the output. There are more void elements in HTML
+                            // but we list only those realistically expected in code display.
+                            if (!tag(child).match(/br|hr|img|input/)) {
+                                result.push({
+                                    event: 'stop',
+                                    offset: offset,
+                                    node: child
+                                });
+                            }
+                        }
+                    }
+                    return offset;
+                })(node, 0);
+                return result;
+            }
+
+            /**
+             * @param {any} original - the original stream
+             * @param {any} highlighted - stream of the highlighted source
+             * @param {string} value - the original source itself
+             */
+            function mergeStreams(original, highlighted, value) {
+                let processed = 0;
+                let result = '';
+                const nodeStack = [];
+
+                function selectStream() {
+                    if (!original.length || !highlighted.length) {
+                        return original.length ? original : highlighted;
+                    }
+                    if (original[0].offset !== highlighted[0].offset) {
+                        return (original[0].offset < highlighted[0].offset) ? original : highlighted;
+                    }
+
+                    /*
+                    To avoid starting the stream just before it should stop the order is
+                    ensured that original always starts first and closes last:
+
+                    if (event1 == 'start' && event2 == 'start')
+                      return original;
+                    if (event1 == 'start' && event2 == 'stop')
+                      return highlighted;
+                    if (event1 == 'stop' && event2 == 'start')
+                      return original;
+                    if (event1 == 'stop' && event2 == 'stop')
+                      return highlighted;
+
+                    ... which is collapsed to:
+                    */
+                    return highlighted[0].event === 'start' ? original : highlighted;
+                }
+
+                /**
+                 * @param {Node} node
+                 */
+                function open(node) {
+                    /** @param {Attr} attr */
+                    function attributeString(attr) {
+                        return ' ' + attr.nodeName + '="' + escapeHTML(attr.value) + '"';
+                    }
+                    // @ts-ignore
+                    result += '<' + tag(node) + [].map.call(node.attributes, attributeString).join('') + '>';
+                }
+
+                /**
+                 * @param {Node} node
+                 */
+                function close(node) {
+                    result += '</' + tag(node) + '>';
+                }
+
+                /**
+                 * @param {Event} event
+                 */
+                function render(event) {
+                    (event.event === 'start' ? open : close)(event.node);
+                }
+
+                while (original.length || highlighted.length) {
+                    let stream = selectStream();
+                    result += escapeHTML(value.substring(processed, stream[0].offset));
+                    processed = stream[0].offset;
+                    if (stream === original) {
+                        /*
+                        On any opening or closing tag of the original markup we first close
+                        the entire highlighted node stack, then render the original tag along
+                        with all the following original tags at the same offset and then
+                        reopen all the tags on the highlighted stack.
+                        */
+                        nodeStack.reverse().forEach(close);
+                        do {
+                            render(stream.splice(0, 1)[0]);
+                            stream = selectStream();
+                        } while (stream === original && stream.length && stream[0].offset === processed);
+                        nodeStack.reverse().forEach(open);
+                    } else {
+                        if (stream[0].event === 'start') {
+                            nodeStack.push(stream[0].node);
+                        } else {
+                            nodeStack.pop();
+                        }
+                        render(stream.splice(0, 1)[0]);
+                    }
+                }
+                return result + escapeHTML(value.substr(processed));
+            }
+
+            return mergeHTMLPlugin;
+
+        }());
+        hljs.addPlugin(mergeHTMLPlugin);
+        // end of copy-pasted code
+
+
         {{ with $.Scratch.Get "hl_languages" }}
         hljs.configure({languages: [{{(delimit . ", ")}}]});
         {{ end }}