Build a virtualised list

• A fixed-height scroll container shows a list of 100,000 generated rows; the DOM contains only the visible rows plus a small overscan (for example 5 above and below)
• Scrolling is smooth: the total scroll height is correct (a spacer or padding for the full list), and visible rows are positioned absolutely or with a translated wrapper
• First version: all rows have a fixed height; the visible range is computed from scrollTop, the container height and the row height
• Second version: variable row heights, with measured heights cached per index and an estimate for rows not yet measured
• Scroll position is preserved and the range is correct after the container is resized; a scrollToIndex(i) function works
• Row components are keyed by index or id and memoised so scrolling only mounts and unmounts the edges

Only after every requirement works.

• Horizontal or grid virtualisation
• "Stick to bottom" behaviour for chat-like lists with new items prepended or appended
• Dynamic height measurement with ResizeObserver