Achieve 100% test coverage on all metrics

- Added tests for uncovered branch conditions
- Fixed process.stdout.columns undefined fallback coverage
- Fixed right arrow key else-if branch coverage
- Now at 100% coverage for statements, branches, functions, and lines
- 61 total tests all passing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: super3

test/mdtail-coverage.test.js

@@ -0,0 +1,120 @@
+const MdTail = require('../lib/mdtail');
+const fs = require('fs');
+const readline = require('readline');
+
+// Mock modules
+jest.mock('fs');
+jest.mock('readline');
+
+describe('MdTail - 100% Branch Coverage', () => {
+  let mdtail;
+  let consoleLogSpy;
+  let consoleClearSpy;
+  let stdoutWriteSpy;
+  let originalColumns;
+
+  beforeEach(() => {
+    mdtail = new MdTail();
+    
+    // Save original columns
+    originalColumns = process.stdout.columns;
+    
+    // Mock console methods
+    consoleLogSpy = jest.spyOn(console, 'log').mockImplementation();
+    consoleClearSpy = jest.spyOn(console, 'clear').mockImplementation();
+    stdoutWriteSpy = jest.spyOn(process.stdout, 'write').mockImplementation();
+    
+    // Mock process.stdin
+    process.stdin.isTTY = true;
+    process.stdin.setRawMode = jest.fn();
+    process.stdin.resume = jest.fn();
+    
+    jest.clearAllMocks();
+  });
+
+  afterEach(() => {
+    // Restore original columns
+    process.stdout.columns = originalColumns;
+    
+    consoleLogSpy.mockRestore();
+    consoleClearSpy.mockRestore();
+    stdoutWriteSpy.mockRestore();
+    jest.clearAllMocks();
+    jest.clearAllTimers();
+    jest.useRealTimers();
+  });
+
+  describe('displayCurrentFile - undefined columns fallback', () => {
+    test('should use default width of 80 when process.stdout.columns is undefined', () => {
+      // Set columns to undefined to test the fallback
+      process.stdout.columns = undefined;
+      
+      mdtail.files = ['/test/file.md'];
+      mdtail.currentTabIndex = 0;
+      fs.readFileSync.mockReturnValue('# Test Content');
+      
+      mdtail.displayCurrentFile();
+      
+      // Should use default width of 80 for the borders
+      expect(consoleLogSpy).toHaveBeenCalledWith('\n' + '═'.repeat(80));
+      expect(consoleLogSpy).toHaveBeenCalledWith('═'.repeat(80) + '\n');
+    });
+  });
+
+  describe('setupKeyboardNavigation - right arrow branch', () => {
+    test('should handle right arrow key with else-if branch', () => {
+      mdtail.files = ['/file1.md', '/file2.md', '/file3.md'];
+      mdtail.currentTabIndex = 0;
+      mdtail.displayCurrentFile = jest.fn();
+      
+      let keypressCallback;
+      process.stdin.on = jest.fn((event, callback) => {
+        if (event === 'keypress') {
+          keypressCallback = callback;
+        }
+      });
+      readline.emitKeypressEvents = jest.fn();
+      
+      mdtail.setupKeyboardNavigation();
+      
+      // First test left arrow
+      keypressCallback('', { name: 'left' });
+      expect(mdtail.currentTabIndex).toBe(2); // Wrapped to last
+      
+      // Reset for right arrow test
+      mdtail.currentTabIndex = 0;
+      mdtail.displayCurrentFile.mockClear();
+      
+      // Now test right arrow (this will hit the else-if branch)
+      keypressCallback('', { name: 'right' });
+      expect(mdtail.currentTabIndex).toBe(1);
+      expect(mdtail.displayCurrentFile).toHaveBeenCalled();
+    });
+    
+    test('should not navigate when key is neither left nor right', () => {
+      mdtail.files = ['/file1.md', '/file2.md'];
+      mdtail.currentTabIndex = 0;
+      mdtail.displayCurrentFile = jest.fn();
+      
+      let keypressCallback;
+      process.stdin.on = jest.fn((event, callback) => {
+        if (event === 'keypress') {
+          keypressCallback = callback;
+        }
+      });
+      readline.emitKeypressEvents = jest.fn();
+      
+      mdtail.setupKeyboardNavigation();
+      
+      // Test with up arrow (should not navigate)
+      keypressCallback('', { name: 'up' });
+      expect(mdtail.currentTabIndex).toBe(0); // Should not change
+      expect(mdtail.displayCurrentFile).not.toHaveBeenCalled();
+      
+      // Test with no key object
+      keypressCallback('a', null);
+      expect(mdtail.currentTabIndex).toBe(0); // Should not change
+      expect(mdtail.displayCurrentFile).not.toHaveBeenCalled();
+    });
+  });
+});