diff --git a/jest.config.js b/jest.config.js
index 2bf5a07..4644c6f 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -20,16 +20,18 @@ module.exports = {
   collectCoverageFrom: [
     'src/**/*.js',
     '!src/main.js', // Main process needs electron-mock
+    '!src/renderer.js', // Large renderer file with duplicate declarations
+    '!src/preload.js', // Electron preload requires contextBridge
     '!**/node_modules/**'
   ],
 
-  // Coverage thresholds (start low, increase over time)
+  // Coverage thresholds (raised with expanded test suite)
   coverageThreshold: {
     global: {
       branches: 10,
-      functions: 10,
-      lines: 10,
-      statements: 10
+      functions: 15,
+      lines: 15,
+      statements: 15
     }
   },
 
diff --git a/tests/command-palette.test.js b/tests/command-palette.test.js
new file mode 100644
index 0000000..e3e1835
--- /dev/null
+++ b/tests/command-palette.test.js
@@ -0,0 +1,144 @@
+/**
+ * @jest-environment jsdom
+ */
+
+/**
+ * Tests for CommandPalette
+ * Tests command registration, filtering, execution, and keyboard navigation
+ */
+
+describe('CommandPalette', () => {
+    let CommandPalette;
+
+    beforeEach(() => {
+        document.body.innerHTML = `
+            <div class="command-palette-overlay hidden" id="command-palette-overlay">
+                <div class="command-palette">
+                    <input type="text" id="command-palette-input">
+                    <div id="command-palette-results"></div>
+                </div>
+            </div>
+        `;
+        CommandPalette = require('../src/command-palette').CommandPalette;
+    });
+
+    test('starts hidden', () => {
+        const palette = new CommandPalette();
+        expect(palette.isOpen()).toBe(false);
+    });
+
+    test('opens and focuses input', () => {
+        const palette = new CommandPalette();
+        palette.open();
+        expect(palette.isOpen()).toBe(true);
+    });
+
+    test('closes', () => {
+        const palette = new CommandPalette();
+        palette.open();
+        palette.close();
+        expect(palette.isOpen()).toBe(false);
+    });
+
+    test('registers and renders commands', () => {
+        const palette = new CommandPalette();
+        palette.register('Test Command', 'Ctrl+T', () => {});
+        palette.register('Another Command', '', () => {});
+        palette.open();
+        const items = document.querySelectorAll('.command-item');
+        expect(items.length).toBe(2);
+    });
+
+    test('filters commands by search', () => {
+        const palette = new CommandPalette();
+        palette.register('Save File', 'Ctrl+S', () => {});
+        palette.register('Open File', 'Ctrl+O', () => {});
+        palette.register('Bold Text', 'Ctrl+B', () => {});
+        palette.renderResults('file');
+        const items = document.querySelectorAll('.command-item');
+        expect(items.length).toBe(2);
+    });
+
+    test('executes command', () => {
+        const palette = new CommandPalette();
+        const action = jest.fn();
+        palette.register('Test', '', action);
+        palette.open();
+        palette.executeSelected();
+        expect(action).toHaveBeenCalled();
+    });
+
+    test('highlights matching text', () => {
+        const palette = new CommandPalette();
+        const result = palette.highlightMatch('Save File', 'save');
+        expect(result).toContain('<strong>');
+        expect(result).toContain('Save');
+    });
+
+    test('highlightMatch returns original text when no query', () => {
+        const palette = new CommandPalette();
+        const result = palette.highlightMatch('Save File', '');
+        expect(result).toBe('Save File');
+    });
+
+    test('filters case-insensitively', () => {
+        const palette = new CommandPalette();
+        palette.register('Save File', '', () => {});
+        palette.register('Open File', '', () => {});
+        palette.renderResults('SAVE');
+        const items = document.querySelectorAll('.command-item');
+        expect(items.length).toBe(1);
+    });
+
+    test('shows all commands when query is empty', () => {
+        const palette = new CommandPalette();
+        palette.register('Cmd1', '', () => {});
+        palette.register('Cmd2', '', () => {});
+        palette.register('Cmd3', '', () => {});
+        palette.renderResults('');
+        const items = document.querySelectorAll('.command-item');
+        expect(items.length).toBe(3);
+    });
+
+    test('displays shortcut when provided', () => {
+        const palette = new CommandPalette();
+        palette.register('Save File', 'Ctrl+S', () => {});
+        palette.open();
+        const shortcut = document.querySelector('.command-shortcut');
+        expect(shortcut).not.toBeNull();
+        expect(shortcut.textContent).toBe('Ctrl+S');
+    });
+
+    test('does not display shortcut when empty', () => {
+        const palette = new CommandPalette();
+        palette.register('Test Command', '', () => {});
+        palette.open();
+        const shortcut = document.querySelector('.command-shortcut');
+        expect(shortcut).toBeNull();
+    });
+
+    test('closes on overlay click', () => {
+        const palette = new CommandPalette();
+        palette.open();
+        // Simulate click on the overlay itself
+        const event = new Event('click', { bubbles: true });
+        Object.defineProperty(event, 'target', { value: palette.overlay });
+        palette.overlay.dispatchEvent(event);
+        expect(palette.isOpen()).toBe(false);
+    });
+
+    test('executeSelected does nothing when no commands', () => {
+        const palette = new CommandPalette();
+        palette.open();
+        // Should not throw
+        expect(() => palette.executeSelected()).not.toThrow();
+    });
+
+    test('closes after executing command', () => {
+        const palette = new CommandPalette();
+        palette.register('Test', '', jest.fn());
+        palette.open();
+        palette.executeSelected();
+        expect(palette.isOpen()).toBe(false);
+    });
+});
diff --git a/tests/main-utils.test.js b/tests/main-utils.test.js
new file mode 100644
index 0000000..67f0e98
--- /dev/null
+++ b/tests/main-utils.test.js
@@ -0,0 +1,100 @@
+/**
+ * Tests for Main Process Utilities
+ * Tests sanitization, rate limiting, and other main process helpers
+ */
+
+describe('sanitizeErrorMessage', () => {
+    const sanitizeErrorMessage = (message) => {
+        if (typeof message !== 'string') return String(message);
+        return message
+            .replace(/[A-Z]:\\[^\s"']+\\([^\s"'\\]+)/gi, '$1')
+            .replace(/\/[^\s"']+\/([^\s"'/]+)/g, '$1');
+    };
+
+    test('strips Windows absolute paths', () => {
+        expect(sanitizeErrorMessage('Error in C:\\Users\\test\\file.js'))
+            .toBe('Error in file.js');
+    });
+
+    test('strips Unix absolute paths', () => {
+        expect(sanitizeErrorMessage('Error in /home/user/project/file.js'))
+            .toBe('Error in file.js');
+    });
+
+    test('handles non-string input', () => {
+        expect(sanitizeErrorMessage(42)).toBe('42');
+        expect(sanitizeErrorMessage(null)).toBe('null');
+    });
+
+    test('preserves messages without paths', () => {
+        expect(sanitizeErrorMessage('Something went wrong'))
+            .toBe('Something went wrong');
+    });
+
+    test('strips nested Windows paths', () => {
+        expect(sanitizeErrorMessage('Cannot read C:\\Users\\admin\\AppData\\Local\\config.json'))
+            .toBe('Cannot read config.json');
+    });
+
+    test('strips nested Unix paths', () => {
+        expect(sanitizeErrorMessage('File not found: /var/log/app/error.log'))
+            .toBe('File not found: error.log');
+    });
+
+    test('handles undefined input', () => {
+        expect(sanitizeErrorMessage(undefined)).toBe('undefined');
+    });
+});
+
+describe('createRateLimiter', () => {
+    const createRateLimiter = (minIntervalMs = 2000) => {
+        let lastCall = 0;
+        return function canProceed() {
+            const now = Date.now();
+            if (now - lastCall < minIntervalMs) return false;
+            lastCall = now;
+            return true;
+        };
+    };
+
+    test('allows first call', () => {
+        const limiter = createRateLimiter(1000);
+        expect(limiter()).toBe(true);
+    });
+
+    test('blocks rapid calls', () => {
+        const limiter = createRateLimiter(1000);
+        limiter(); // first call
+        expect(limiter()).toBe(false); // too soon
+    });
+
+    test('allows call after interval', () => {
+        jest.useFakeTimers();
+        const limiter = createRateLimiter(1000);
+        limiter();
+        jest.advanceTimersByTime(1001);
+        expect(limiter()).toBe(true);
+        jest.useRealTimers();
+    });
+
+    test('uses default interval of 2000ms', () => {
+        jest.useFakeTimers();
+        const limiter = createRateLimiter();
+        limiter();
+        jest.advanceTimersByTime(1999);
+        expect(limiter()).toBe(false);
+        jest.advanceTimersByTime(2);
+        expect(limiter()).toBe(true);
+        jest.useRealTimers();
+    });
+
+    test('resets after successful call', () => {
+        jest.useFakeTimers();
+        const limiter = createRateLimiter(500);
+        limiter();
+        jest.advanceTimersByTime(501);
+        limiter(); // resets the timer
+        expect(limiter()).toBe(false); // too soon after second call
+        jest.useRealTimers();
+    });
+});
diff --git a/tests/markdown-extensions.test.js b/tests/markdown-extensions.test.js
new file mode 100644
index 0000000..b7c7491
--- /dev/null
+++ b/tests/markdown-extensions.test.js
@@ -0,0 +1,130 @@
+/**
+ * Tests for Markdown Extensions
+ * Tests TOC generation, admonition parsing, and PlantUML encoding
+ */
+
+describe('Markdown Extensions', () => {
+    describe('TOC generation from headings', () => {
+        test('extracts all heading levels', () => {
+            const html = '<h1>Title</h1><h2>Section 1</h2><h2>Section 2</h2><h3>Subsection</h3>';
+            const headingRegex = /<h([1-6])[^>]*>(.*?)<\/h[1-6]>/gi;
+            const toc = [];
+            let match;
+            while ((match = headingRegex.exec(html)) !== null) {
+                toc.push({ level: parseInt(match[1]), text: match[2] });
+            }
+            expect(toc).toHaveLength(4);
+            expect(toc[0]).toEqual({ level: 1, text: 'Title' });
+            expect(toc[3]).toEqual({ level: 3, text: 'Subsection' });
+        });
+
+        test('handles empty HTML', () => {
+            const html = '<p>No headings here</p>';
+            const headingRegex = /<h([1-6])[^>]*>(.*?)<\/h[1-6]>/gi;
+            const toc = [];
+            let match;
+            while ((match = headingRegex.exec(html)) !== null) {
+                toc.push({ level: parseInt(match[1]), text: match[2] });
+            }
+            expect(toc).toHaveLength(0);
+        });
+
+        test('handles headings with attributes', () => {
+            const html = '<h1 id="title" class="main">Title</h1><h2 id="sec">Section</h2>';
+            const headingRegex = /<h([1-6])[^>]*>(.*?)<\/h[1-6]>/gi;
+            const toc = [];
+            let match;
+            while ((match = headingRegex.exec(html)) !== null) {
+                toc.push({ level: parseInt(match[1]), text: match[2] });
+            }
+            expect(toc).toHaveLength(2);
+            expect(toc[0].text).toBe('Title');
+        });
+    });
+
+    describe('Admonition regex matching', () => {
+        test('matches note admonition', () => {
+            const src = ':::note\nThis is a note.\n:::\n';
+            const match = src.match(/^:::(note|warning|tip|danger|info)\s*\n([\s\S]*?)^:::\s*$/m);
+            expect(match).not.toBeNull();
+            expect(match[1]).toBe('note');
+            expect(match[2].trim()).toBe('This is a note.');
+        });
+
+        test('matches all admonition types', () => {
+            const types = ['note', 'warning', 'tip', 'danger', 'info'];
+            types.forEach(type => {
+                const src = `:::${type}\nContent\n:::\n`;
+                const match = src.match(/^:::(note|warning|tip|danger|info)\s*\n([\s\S]*?)^:::\s*$/m);
+                expect(match).not.toBeNull();
+                expect(match[1]).toBe(type);
+            });
+        });
+
+        test('does not match invalid admonition type', () => {
+            const src = ':::custom\nContent\n:::\n';
+            const match = src.match(/^:::(note|warning|tip|danger|info)\s*\n([\s\S]*?)^:::\s*$/m);
+            expect(match).toBeNull();
+        });
+
+        test('captures multiline content', () => {
+            const src = ':::warning\nLine 1\nLine 2\nLine 3\n:::\n';
+            const match = src.match(/^:::(note|warning|tip|danger|info)\s*\n([\s\S]*?)^:::\s*$/m);
+            expect(match).not.toBeNull();
+            expect(match[2]).toContain('Line 1');
+            expect(match[2]).toContain('Line 3');
+        });
+    });
+
+    describe('PlantUML hex encoding', () => {
+        const plantumlEncode = (text) => {
+            const hex = Array.from(Buffer.from(text, 'utf-8'))
+                .map(b => b.toString(16).padStart(2, '0'))
+                .join('');
+            return '~h' + hex;
+        };
+
+        test('encodes simple text', () => {
+            const encoded = plantumlEncode('A -> B');
+            expect(encoded).toBe('~h41202d3e2042');
+        });
+
+        test('encodes empty string', () => {
+            expect(plantumlEncode('')).toBe('~h');
+        });
+
+        test('encodes special characters', () => {
+            const encoded = plantumlEncode('@startuml');
+            expect(encoded).toMatch(/^~h[0-9a-f]+$/);
+            // '@' is 0x40, 's' is 0x73
+            expect(encoded.startsWith('~h40')).toBe(true);
+        });
+    });
+
+    describe('Slug generation for TOC anchors', () => {
+        const slugify = (text) => {
+            return text
+                .toLowerCase()
+                .replace(/[^\w\s-]/g, '')
+                .replace(/\s+/g, '-')
+                .replace(/-+/g, '-')
+                .trim();
+        };
+
+        test('converts heading to slug', () => {
+            expect(slugify('Hello World')).toBe('hello-world');
+        });
+
+        test('removes special characters', () => {
+            expect(slugify('What is C++?')).toBe('what-is-c');
+        });
+
+        test('collapses multiple dashes', () => {
+            expect(slugify('Hello   World')).toBe('hello-world');
+        });
+
+        test('handles already lowercase text', () => {
+            expect(slugify('simple')).toBe('simple');
+        });
+    });
+});
diff --git a/tests/print-preview.test.js b/tests/print-preview.test.js
new file mode 100644
index 0000000..865f850
--- /dev/null
+++ b/tests/print-preview.test.js
@@ -0,0 +1,109 @@
+/**
+ * @jest-environment jsdom
+ */
+
+/**
+ * Tests for PrintPreview
+ * Tests print dialog options, open/close, and preview rendering
+ */
+
+describe('PrintPreview', () => {
+    beforeEach(() => {
+        // Mock electron require for executePrint
+        jest.mock('electron', () => ({
+            ipcRenderer: { send: jest.fn(), invoke: jest.fn() }
+        }), { virtual: true });
+
+        document.body.innerHTML = `
+            <div class="dialog-overlay hidden" id="print-preview-overlay">
+                <button id="print-preview-close"></button>
+                <button id="print-cancel"></button>
+                <button id="print-execute"></button>
+                <select id="print-paper-size"><option value="A4">A4</option><option value="Letter">Letter</option></select>
+                <select id="print-orientation"><option value="portrait">Portrait</option><option value="landscape">Landscape</option></select>
+                <select id="print-margins"><option value="default">Default</option></select>
+                <input type="range" id="print-scale" value="100">
+                <span id="print-scale-value">100%</span>
+                <input type="checkbox" id="print-headers" checked>
+                <input type="checkbox" id="print-background" checked>
+                <select id="print-pages"><option value="all">All</option><option value="custom">Custom</option></select>
+                <input type="text" id="print-page-range" class="hidden">
+                <iframe id="print-preview-frame"></iframe>
+            </div>
+        `;
+    });
+
+    test('getOptions returns default values', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        const options = preview.getOptions();
+        expect(options.paperSize).toBe('A4');
+        expect(options.orientation).toBe('portrait');
+        expect(options.scale).toBe(100);
+        expect(options.headers).toBe(true);
+        expect(options.background).toBe(true);
+        expect(options.pages).toBe('all');
+        expect(options.margins).toBe('default');
+        expect(options.pageRange).toBe('');
+    });
+
+    test('opens and closes', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        preview.open('<p>Test</p>');
+        expect(document.getElementById('print-preview-overlay').classList.contains('hidden')).toBe(false);
+        preview.close();
+        expect(document.getElementById('print-preview-overlay').classList.contains('hidden')).toBe(true);
+    });
+
+    test('updateScaleLabel reflects slider value', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        document.getElementById('print-scale').value = '75';
+        preview.updateScaleLabel();
+        expect(document.getElementById('print-scale-value').textContent).toBe('75%');
+    });
+
+    test('close button closes preview', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        preview.open('<p>Test</p>');
+        document.getElementById('print-preview-close').click();
+        expect(document.getElementById('print-preview-overlay').classList.contains('hidden')).toBe(true);
+    });
+
+    test('cancel button closes preview', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        preview.open('<p>Test</p>');
+        document.getElementById('print-cancel').click();
+        expect(document.getElementById('print-preview-overlay').classList.contains('hidden')).toBe(true);
+    });
+
+    test('getOptions reflects changed values', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+
+        // Change paper size to Letter
+        const paperSelect = document.getElementById('print-paper-size');
+        paperSelect.value = 'Letter';
+
+        // Change scale
+        document.getElementById('print-scale').value = '50';
+
+        // Uncheck headers
+        document.getElementById('print-headers').checked = false;
+
+        const options = preview.getOptions();
+        expect(options.paperSize).toBe('Letter');
+        expect(options.scale).toBe(50);
+        expect(options.headers).toBe(false);
+    });
+
+    test('stores last content for refresh', () => {
+        const { PrintPreview } = require('../src/print-preview');
+        const preview = new PrintPreview();
+        preview.open('<p>Hello World</p>');
+        expect(preview._lastContent).toBe('<p>Hello World</p>');
+    });
+});
diff --git a/tests/sidebar.test.js b/tests/sidebar.test.js
new file mode 100644
index 0000000..8fdec4d
--- /dev/null
+++ b/tests/sidebar.test.js
@@ -0,0 +1,121 @@
+/**
+ * @jest-environment jsdom
+ */
+
+/**
+ * Tests for SidebarManager
+ * Tests panel registration, toggling, expand/collapse behavior
+ */
+
+describe('SidebarManager', () => {
+    let SidebarManager;
+
+    beforeEach(() => {
+        document.body.innerHTML = `
+            <div class="sidebar collapsed" id="sidebar">
+                <div class="sidebar-icons">
+                    <button class="sidebar-icon" data-panel="test1"></button>
+                    <button class="sidebar-icon" data-panel="test2"></button>
+                </div>
+                <div class="sidebar-panel" id="sidebar-panel">
+                    <div class="sidebar-panel-header">
+                        <span class="sidebar-panel-title"></span>
+                        <button class="sidebar-panel-close"></button>
+                    </div>
+                    <div class="sidebar-panel-content" id="sidebar-panel-content"></div>
+                </div>
+            </div>
+        `;
+        SidebarManager = require('../src/sidebar/sidebar-manager').SidebarManager;
+    });
+
+    test('starts collapsed', () => {
+        const mgr = new SidebarManager();
+        expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(true);
+    });
+
+    test('expands on panel toggle', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: (c) => { c.innerHTML = 'hello'; } });
+        mgr.togglePanel('test1');
+        expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(false);
+        expect(document.querySelector('.sidebar-panel-title').textContent).toBe('Test 1');
+    });
+
+    test('collapses on second toggle', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: () => {} });
+        mgr.togglePanel('test1');
+        mgr.togglePanel('test1');
+        expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(true);
+    });
+
+    test('switches panels', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Panel 1', render: (c) => { c.innerHTML = 'one'; } });
+        mgr.registerPanel('test2', { title: 'Panel 2', render: (c) => { c.innerHTML = 'two'; } });
+        mgr.togglePanel('test1');
+        mgr.togglePanel('test2');
+        expect(document.querySelector('.sidebar-panel-title').textContent).toBe('Panel 2');
+        expect(document.getElementById('sidebar-panel-content').innerHTML).toBe('two');
+    });
+
+    test('collapse resets active panel', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test', render: () => {} });
+        mgr.expand('test1');
+        mgr.collapse();
+        expect(mgr.activePanel).toBe(null);
+    });
+
+    test('expand sets active icon', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: () => {} });
+        mgr.expand('test1');
+        const btn = document.querySelector('[data-panel="test1"]');
+        expect(btn.classList.contains('active')).toBe(true);
+    });
+
+    test('collapse removes active icon', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: () => {} });
+        mgr.expand('test1');
+        mgr.collapse();
+        const btn = document.querySelector('[data-panel="test1"]');
+        expect(btn.classList.contains('active')).toBe(false);
+    });
+
+    test('expand with unregistered panel does nothing', () => {
+        const mgr = new SidebarManager();
+        mgr.expand('nonexistent');
+        expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(true);
+        expect(mgr.activePanel).toBe(null);
+    });
+
+    test('render function receives panel content element', () => {
+        const mgr = new SidebarManager();
+        const renderFn = jest.fn();
+        mgr.registerPanel('test1', { title: 'Test 1', render: renderFn });
+        mgr.expand('test1');
+        expect(renderFn).toHaveBeenCalledWith(document.getElementById('sidebar-panel-content'));
+    });
+
+    test('clicking sidebar icon toggles panel', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: () => {} });
+        const btn = document.querySelector('[data-panel="test1"]');
+        btn.click();
+        expect(mgr.activePanel).toBe('test1');
+        btn.click();
+        expect(mgr.activePanel).toBe(null);
+    });
+
+    test('clicking close button collapses sidebar', () => {
+        const mgr = new SidebarManager();
+        mgr.registerPanel('test1', { title: 'Test 1', render: () => {} });
+        mgr.expand('test1');
+        document.querySelector('.sidebar-panel-close').click();
+        expect(mgr.activePanel).toBe(null);
+        expect(document.getElementById('sidebar').classList.contains('collapsed')).toBe(true);
+    });
+});