On first mount (and on session switch, via a sessionId key remount) the
chart scrolls to the right edge so the latest steps are visible
immediately; afterwards it only sticks to the end while the user is near
it, so scrolling back into earlier turns/steps is never fought. The edge
fades follow the scroll position as before. The render test's fake React
now keeps useRef stable across renders (like React), which is what the
effect needs to see the container.
bowenliang123
d2d0510ea0