feat: add keyboard navigation controller

Author: maribethb

core/keyboard_navigation_controller.ts

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * The KeyboardNavigationController handles coordinating Blockly-wide
+ * keyboard navigation behavior, such as registering shortcut handlers
+ * or enabling/disabling full cursor visualization.
+ */
+class KeyboardNavigationController {
+  /** Whether the user is actively using keyboard navigation. */
+  private isActive = false;
+  /** Css class name added to body if keyboard nav is active. */
+  private activeClassName = 'blocklyActiveKeyboardNav';
+
+  /**
+   * Sets whether we think the user is actively using keyboard navigation.
+   * 
+   * If they are, we'll apply a css class to the entire page so that
+   * focused items can apply additional styling for keyboard users.
+   * 
+   * @param isUsing
+   */
+  setIsActive(isUsing: boolean = true) {
+    this.isActive = isUsing;
+    this.updateActiveVisualization();
+  }
+
+  /**
+   * @returns true if we think the user is actively using keyboard navigation
+   * (e.g., has recently taken some action that is only relevant to keyboard users)
+   */
+  getIsActive(): boolean {
+    return this.isActive;
+  }
+
+  /** Adds or removes the css class that indicates keyboard navigation is active. */
+  private updateActiveVisualization() {
+    if (this.isActive) {
+      document.body.classList.add(this.activeClassName);
+    } else {
+      document.body.classList.remove(this.activeClassName);
+    }
+  }
+}
+
+/** Singleton instance of the keyboard navigation controller. */
+export const keyboardNavigationController = new KeyboardNavigationController();