feat: add CodeMirror 6 editor with Emmet support

- Replace textarea with CodeMirror 6 for syntax highlighting
- Add Emmet abbreviation expansion (Tab to expand)
- Support HTML and CSS language modes with autocomplete
- Add dark theme matching app design
- Tab indentation with Shift-Tab for outdent
- Update help modal with Emmet shortcuts

src/app.js

@@ -1,4 +1,5 @@
 import { LessonEngine } from "./impl/LessonEngine.js";
+import { CodeEditor } from "./impl/CodeEditor.js";
 import { renderLesson, renderModuleList, renderLevelIndicator, updateActiveLessonInSidebar } from "./helpers/renderer.js";
 import { loadModules } from "./config/lessons.js";
 
@@ -56,6 +57,10 @@ const elements = {
 // Initialize the lesson engine - now the single source of truth
 const lessonEngine = new LessonEngine();
 
+// Code editor instance (initialized later)
+let codeEditor = null;
+let currentMode = "css";
+
 // ================= SIDEBAR FUNCTIONS =================
 
 function openSidebar() {
@@ -214,27 +219,34 @@ function resetSuccessIndicators() {
 }
 
 function updateEditorForMode(mode) {
-	const codeInput = elements.codeInput;
 	const editorLabel = document.querySelector(".editor-label");
 
 	const modeConfig = {
 		html: {
-			placeholder: "Write your HTML here (e.g., <p>Hello World</p>)",
-			label: "HTML Editor"
+			placeholder: "Type HTML here... Try: nav>ul>li*3 then press Tab",
+			label: "HTML Editor",
+			cmMode: "html"
 		},
 		tailwind: {
 			placeholder: "Enter Tailwind classes (e.g., bg-blue-500 text-white p-4)",
-			label: "Tailwind Classes"
+			label: "Tailwind Classes",
+			cmMode: "css"
 		},
 		css: {
 			placeholder: "Enter your CSS code here...",
-			label: "CSS Editor"
+			label: "CSS Editor",
+			cmMode: "css"
 		}
 	};
 
 	const config = modeConfig[mode] || modeConfig.css;
-	codeInput.placeholder = config.placeholder;
 	if (editorLabel) editorLabel.textContent = config.label;
+
+	// Update CodeMirror mode if needed
+	if (codeEditor && currentMode !== config.cmMode) {
+		currentMode = config.cmMode;
+		codeEditor.setMode(config.cmMode);
+	}
 }
 
 function loadCurrentLesson() {
@@ -269,13 +281,15 @@ function loadCurrentLesson() {
 		elements.taskInstruction,
 		elements.previewArea,
 		null, // editorPrefix no longer used
-		elements.codeInput,
+		null, // codeInput no longer used (using CodeMirror)
 		null, // editorSuffix no longer used
 		lesson
 	);
 
-	// Set user code in input
-	elements.codeInput.value = engineState.userCode;
+	// Set user code in CodeMirror
+	if (codeEditor) {
+		codeEditor.setValue(engineState.userCode);
+	}
 
 	// Reset validation indicators
 	elements.validationIndicators.innerHTML = "";
@@ -312,25 +326,19 @@ function loadCurrentLesson() {
 	updateProgressDisplay();
 
 	// Focus on the code editor
-	elements.codeInput.focus();
+	if (codeEditor) {
+		codeEditor.focus();
+	}
 
 	// Render the expected/solution preview
 	lessonEngine.renderExpectedPreview();
-
-	// Setup live preview
-	setupLivePreview();
 }
 
 // ================= LIVE PREVIEW =================
 
 let previewTimer = null;
 
-function setupLivePreview() {
-	elements.codeInput.removeEventListener("input", handleUserInput);
-	elements.codeInput.addEventListener("input", handleUserInput);
-}
-
-function handleUserInput() {
+function handleEditorChange(code) {
 	if (previewTimer) {
 		clearTimeout(previewTimer);
 	}
@@ -369,7 +377,7 @@ function prevLesson() {
 // ================= CODE EXECUTION =================
 
 function runCode() {
-	const userCode = elements.codeInput.value;
+	const userCode = codeEditor ? codeEditor.getValue() : "";
 
 	// Rotate the Run button icon
 	const runButtonImg = document.querySelector("#run-btn img");
@@ -460,9 +468,17 @@ function showHelp() {
 		<ul>
 			<li>Click "Show Expected" to see the target result</li>
 			<li>Your progress is saved automatically</li>
-			<li>Use Tab for indentation</li>
 			<li>Ctrl+Enter runs your code</li>
 		</ul>
+
+		<h4>Emmet Shortcuts (HTML mode)</h4>
+		<p>Type abbreviations and press Tab to expand:</p>
+		<ul>
+			<li><kbd>div.container</kbd> → div with class</li>
+			<li><kbd>ul>li*5</kbd> → ul with 5 li children</li>
+			<li><kbd>nav>ul>li*3>a</kbd> → nested structure</li>
+			<li><kbd>p{Hello}</kbd> → p with text content</li>
+		</ul>
 	`;
 
 	elements.modalContainer.classList.remove("hidden");
@@ -501,24 +517,35 @@ function closeModal() {
 	elements.modalContainer.classList.add("hidden");
 }
 
-// ================= KEYBOARD HANDLERS =================
-
-function handleTabKey(e) {
-	if (e.key === "Tab") {
-		e.preventDefault();
-
-		const start = e.target.selectionStart;
-		const end = e.target.selectionEnd;
-
-		e.target.value = e.target.value.substring(0, start) + "  " + e.target.value.substring(end);
-		e.target.selectionStart = e.target.selectionEnd = start + 2;
-	}
-}
-
 // ================= INITIALIZATION =================
 
+function initCodeEditor() {
+	const container = elements.editorContent;
+	if (!container) return;
+
+	// Remove the textarea - CodeMirror will replace it
+	const textarea = container.querySelector("textarea");
+	if (textarea) {
+		textarea.remove();
+	}
+
+	// Initialize CodeMirror
+	codeEditor = new CodeEditor(container, {
+		mode: currentMode,
+		placeholder: "Type your code here...",
+		onChange: handleEditorChange
+	});
+
+	codeEditor.init("");
+}
+
 function init() {
 	loadUserSettings();
+
+	// Initialize CodeMirror editor
+	initCodeEditor();
+
+	// Load modules after editor is ready
 	initializeModules().catch(console.error);
 
 	// Sidebar controls
@@ -545,10 +572,9 @@ function init() {
 		saveUserSettings();
 	});
 
-	// Editor interactions
-	elements.codeInput.addEventListener("keydown", handleTabKey);
+	// Click on editor content to focus CodeMirror
 	elements.editorContent?.addEventListener("click", () => {
-		elements.codeInput.focus();
+		if (codeEditor) codeEditor.focus();
 	});
 
 	// Keyboard shortcuts

src/impl/CodeEditor.js

@@ -0,0 +1,194 @@
+/**
+ * CodeEditor - CodeMirror 6 wrapper with Emmet support
+ */
+import { EditorState, Prec } from "@codemirror/state";
+import { EditorView, keymap, placeholder } from "@codemirror/view";
+import { defaultKeymap, indentWithTab, indentMore, indentLess } from "@codemirror/commands";
+import { html } from "@codemirror/lang-html";
+import { css } from "@codemirror/lang-css";
+import { autocompletion } from "@codemirror/autocomplete";
+import { abbreviationTracker, expandAbbreviation } from "@emmetio/codemirror6-plugin";
+
+// Dark theme matching our editor
+const editorTheme = EditorView.theme({
+	"&": {
+		height: "100%",
+		fontSize: "14px",
+		backgroundColor: "#1e1e1e"
+	},
+	".cm-content": {
+		fontFamily: "'JetBrains Mono', 'Fira Code', monospace",
+		padding: "12px 0",
+		caretColor: "#fff",
+		color: "#d4d4d4"
+	},
+	".cm-line": {
+		padding: "0 12px"
+	},
+	"&.cm-focused .cm-cursor": {
+		borderLeftColor: "#fff"
+	},
+	"&.cm-focused .cm-selectionBackground, .cm-selectionBackground": {
+		backgroundColor: "#3a3d41"
+	},
+	".cm-activeLine": {
+		backgroundColor: "#2a2d2e"
+	},
+	".cm-gutters": {
+		backgroundColor: "#1e1e1e",
+		color: "#858585",
+		border: "none",
+		paddingRight: "8px"
+	},
+	".cm-activeLineGutter": {
+		backgroundColor: "#2a2d2e"
+	},
+	// Syntax highlighting
+	".cm-keyword": { color: "#569cd6" },
+	".cm-tagName": { color: "#569cd6" },
+	".cm-attributeName": { color: "#9cdcfe" },
+	".cm-attributeValue": { color: "#ce9178" },
+	".cm-string": { color: "#ce9178" },
+	".cm-comment": { color: "#6a9955" },
+	".cm-propertyName": { color: "#9cdcfe" },
+	".cm-number": { color: "#b5cea8" },
+	".cm-unit": { color: "#b5cea8" },
+	".cm-variableName": { color: "#9cdcfe" },
+	// Autocomplete
+	".cm-tooltip": {
+		backgroundColor: "#252526",
+		border: "1px solid #454545"
+	},
+	".cm-tooltip-autocomplete": {
+		"& > ul > li": {
+			padding: "4px 8px"
+		},
+		"& > ul > li[aria-selected]": {
+			backgroundColor: "#094771",
+			color: "#fff"
+		}
+	}
+}, { dark: true });
+
+export class CodeEditor {
+	constructor(container, options = {}) {
+		this.container = container;
+		this.options = options;
+		this.view = null;
+		this.mode = options.mode || "css";
+		this.onChange = options.onChange || (() => {});
+	}
+
+	/**
+	 * Initialize the editor
+	 */
+	init(initialValue = "") {
+		// Clear container
+		this.container.innerHTML = "";
+
+		// Get language extension based on mode
+		const langExtension = this.mode === "html" ? html() : css();
+
+		// Build extensions array
+		const extensions = [
+			langExtension,
+			editorTheme,
+			// Emmet abbreviation tracking
+			abbreviationTracker(),
+			// High priority keymap for Emmet
+			Prec.highest(keymap.of([
+				{
+					key: "Tab",
+					run: expandAbbreviation
+				}
+			])),
+			// Standard keymaps
+			keymap.of([
+				{ key: "Tab", run: indentMore },
+				{ key: "Shift-Tab", run: indentLess },
+				...defaultKeymap
+			]),
+			autocompletion({
+				activateOnTyping: true,
+				maxRenderedOptions: 10
+			}),
+			EditorView.updateListener.of((update) => {
+				if (update.docChanged) {
+					this.onChange(this.getValue());
+				}
+			}),
+			EditorView.lineWrapping
+		];
+
+		// Add placeholder if provided
+		if (this.options.placeholder) {
+			extensions.push(placeholder(this.options.placeholder));
+		}
+
+		// Create editor state
+		const state = EditorState.create({
+			doc: initialValue,
+			extensions
+		});
+
+		// Create editor view
+		this.view = new EditorView({
+			state,
+			parent: this.container
+		});
+
+		return this;
+	}
+
+	/**
+	 * Get current editor value
+	 */
+	getValue() {
+		return this.view ? this.view.state.doc.toString() : "";
+	}
+
+	/**
+	 * Set editor value
+	 */
+	setValue(value) {
+		if (!this.view) return;
+
+		this.view.dispatch({
+			changes: {
+				from: 0,
+				to: this.view.state.doc.length,
+				insert: value
+			}
+		});
+	}
+
+	/**
+	 * Set editor mode (html or css)
+	 */
+	setMode(mode) {
+		if (this.mode === mode) return;
+
+		this.mode = mode;
+		const currentValue = this.getValue();
+		this.init(currentValue);
+	}
+
+	/**
+	 * Focus the editor
+	 */
+	focus() {
+		if (this.view) {
+			this.view.focus();
+		}
+	}
+
+	/**
+	 * Destroy the editor
+	 */
+	destroy() {
+		if (this.view) {
+			this.view.destroy();
+			this.view = null;
+		}
+	}
+}

src/main.css

@@ -264,9 +264,23 @@ code, kbd {
 .editor-content {
 	flex: 1;
 	display: flex;
+	flex-direction: column;
 	min-height: 0;
+	background: var(--editor-bg);
+	overflow: hidden;
 }
 
+/* CodeMirror container styles */
+.editor-content .cm-editor {
+	flex: 1;
+	height: 100%;
+}
+
+.editor-content .cm-scroller {
+	overflow: auto;
+}
+
+/* Legacy textarea (fallback) */
 .code-input {
 	flex: 1;
 	width: 100%;