UI Atoms guides

This first batch publishes 2 of the 14 upstream UI Atoms guides; the rest are converted and will be released in later batches.

• Build a Scroll Progress Indicator — Create a scroll progress bar, stepped progress tracker, or any visual affordance that communicates how far through a page or section the user has scrolled.
• Position Aware Tooltips — Build tooltips and popovers with directional arrows (or other visual styling) that automatically point the correct way when the element flips to a fallback position.