Prevent keyboard modifier events from scrolling down

Fixes #363

src/utils/Keyboard.test.ts

@@ -0,0 +1,21 @@
+import { assert } from 'chai';
+import * as Keyboard from './Keyboard';
+
+describe('Keyboard', () => {
+  describe('isModifierOnlyKeyboardEvent', () => {
+    it('should return true when only modifier keys are used', () => {
+      // Note that KeyboardEvent.keyCode is deprecated but we're using it to improve browser
+      // compatibility. This helper returns the `any` type because KeyboardEvent doesn't exist under
+      // NodeJS.
+      function createEvent(keyCode: number): any {
+        return { keyCode };
+      }
+      assert.isTrue(Keyboard.isModifierOnlyKeyboardEvent(createEvent(16)));
+      assert.isTrue(Keyboard.isModifierOnlyKeyboardEvent(createEvent(17)));
+      assert.isTrue(Keyboard.isModifierOnlyKeyboardEvent(createEvent(18)));
+      assert.isTrue(Keyboard.isModifierOnlyKeyboardEvent(createEvent(91)));
+      assert.isFalse(Keyboard.isModifierOnlyKeyboardEvent(createEvent(19)));
+      assert.isFalse(Keyboard.isModifierOnlyKeyboardEvent(createEvent(90)));
+    });
+  });
+});

src/utils/Keyboard.ts

@@ -0,0 +1,22 @@
+/**
+ * xterm.js: xterm, in the browser
+ * Copyright (c) 2016, SourceLair Private Company <www.sourcelair.com> (MIT License)
+ */
+
+/**
+ * Keyboard utilities module. This module contains utilities for dealing with keyboard interaction.
+ * @module xterm/utils/Keyboard
+ */
+
+/**
+ * Gets whether a KeyboardEvent is made up entirely of modifier keys.
+ *
+ * @param event The event to check.
+ * @return Whether the KeyboardEvent is made up entirely of modifier keys.
+ */
+export function isModifierOnlyKeyboardEvent(event: KeyboardEvent): boolean {
+  return event.keyCode === 16 || // Shift
+      event.keyCode === 17 || // Control
+      event.keyCode === 18 || // Alt
+      event.keyCode === 91; // Meta
+}