refactor: trigger range fetching before items reach viewport#9480
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
`_updateScrollerItem` ran the fetch-range load whenever a column tree existed, triggering loads during non-scroll updates. Gate it on the scrolling state (`overscrollBehavior === 'auto'`) so the load only happens while the user is actually scrolling.
The web component renamed `_updateScrollerItem` to `__updateVirtualizerElement` and no longer toggles the `overscrollBehavior` style while scrolling. Hook the new method and use the scroller's `scrolling` attribute instead.
The fetch range is now clamped to the last valid index, so at the end of the grid the connector requests only the last existing page (count 50) instead of also a page past the end (count 100). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Clamping the range after expanding it to page boundaries could pull the end index off a page boundary near the last page. Clamp the buffer expansion instead, so the final range stays page-aligned.
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The grid web component fires a page request to the data provider only when a row from that page gets rendered in the DOM. Combined with the limited size of the viewport buffer, this often leaves the grid briefly showing lots of blank rows even when scrolling at a relatively slow speed, since pages can't load fast enough to keep up. Then, when the response arrives, many rendered rows get updated at once, triggering a heavy synchronous DOM update that blocks the event loop and has a negative impact on FPS. This effect got even more noticeable after vaadin/web-components#11196, which reduced the number of rendered rows outside the viewport. So while render time metrics improved after that fix, scroll frame time metrics regressed.
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