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* @module UIUtilities/NavigationEnhancement
* @category User Experience - Accessibility & Navigation
*
* @description
* **Back to Top Navigation Button - Accessibility-First Design**
*
* Minimal JavaScript utility providing smooth scroll-to-top functionality
* with comprehensive accessibility support. Implements WCAG 2.1 AA compliant
* navigation enhancement for long-form intelligence reports and dashboards.
*
* ## Accessibility Features
*
* **WCAG 2.1 AA Compliance**:
* - **Keyboard Navigation**: Full keyboard accessibility (Enter/Space)
* - **Screen Reader**: ARIA labels, semantic HTML
* - **Reduced Motion**: Respects `prefers-reduced-motion` media query
* - **Focus Indicators**: Visible focus states for keyboard users
* - **High Contrast**: Works with high contrast mode
*
* ## User Experience Pattern
*
* **Progressive Enhancement**:
* - Button hidden until user scrolls 300px (intentional navigation)
* - Fade-in animation (respects motion preferences)
* - Fixed position in bottom-right corner (non-intrusive)
* - Smooth scroll behavior (instant scroll for reduced motion users)
*
* ## Motion Sensitivity
*
* **Adaptive Behavior**:
* ```javascript
* const prefersReducedMotion = matchMedia('(prefers-reduced-motion: reduce)').matches;
*
* if (prefersReducedMotion) {
* window.scrollTo({ top: 0, behavior: 'auto' }); // Instant
* } else {
* window.scrollTo({ top: 0, behavior: 'smooth' }); // Animated
* }
* ```
*
* ## Performance Characteristics
*
* **Optimized Event Handling**:
* - Scroll event throttling: 100ms (passive listener)
* - No DOM manipulation during scroll (classList only)
* - CSS transitions for animations (GPU-accelerated)
* - Minimal JavaScript execution (<1ms per scroll event)
*
* ## Security Considerations
*
* @security Minimal risk - Pure client-side UI enhancement
* No data processing, no network requests, no user input handling
*
* ## Integration
*
* **HTML Structure Required**:
* ```html
* <button id="back-to-top" aria-label="Back to top" class="back-to-top-btn">
* ↑
* </button>
* ```
*
* **CSS Classes Expected**:
* - `.back-to-top-btn` - Base styling
* - `.visible` - Show state (opacity: 1)
* - Transition properties for smooth appearance
*
* ## Browser Support
*
* **Modern Browsers** (ES6+):
* - Chrome/Edge 90+
* - Firefox 88+
* - Safari 14+
* - Graceful degradation for older browsers
*
* ## Use Cases
*
* **Deployed On**:
* - Long intelligence reports (news articles)
* - Multi-section dashboards (risk, election, ministry)
* - Politician profiles (extensive biographical data)
* - Historical analysis pages (50+ years of data)
*
* @author Hack23 AB - UX Engineering
* @license Apache-2.0
* @version 1.0.0
* @since 2024
*
* @accessibility WCAG 2.1 AA compliant
* @performance <1ms per scroll event, GPU-accelerated animations
*/
(function() {
'use strict';
/**
* Ensure the back-to-top button markup exists in the DOM.
*
* On news articles and root `index*.html` pages the markup is authored
* directly in the HTML. On pages that load this script without the
* markup (dashboard/index*.html, politician-dashboard*.html), we inject
* a semantically identical button as a progressive enhancement so the
* feature works everywhere without requiring each template to emit
* boilerplate separately.
*
* The injected button uses the same id/class/aria-label contract as
* the authored markup so CSS rules in `styles.css` (.back-to-top,
* .back-to-top.visible) apply uniformly.
*/
function ensureButton() {
var existing = document.getElementById('back-to-top');
if (existing) return existing;
if (!document.body) return null; // called before <body> parsed — unexpected
var btn = document.createElement('button');
btn.id = 'back-to-top';
btn.className = 'back-to-top';
btn.type = 'button';
btn.setAttribute('aria-label', 'Back to top');
btn.setAttribute('title', 'Back to top');
// Inline the arrow glyph used by the authored markup for visual parity.
btn.innerHTML = '<span aria-hidden="true">↑</span>';
document.body.appendChild(btn);
return btn;
}
var backToTopButton = ensureButton();
if (backToTopButton) {
// Show/hide button based on scroll position
window.addEventListener('scroll', function() {
if (window.pageYOffset > 300) {
backToTopButton.classList.add('visible');
} else {
backToTopButton.classList.remove('visible');
}
});
// Smooth scroll to top with reduced motion support
backToTopButton.addEventListener('click', function() {
var prefersReducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
window.scrollTo({
top: 0,
behavior: prefersReducedMotion ? 'auto' : 'smooth'
});
});
}
})();
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