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Pull request overview
This PR optimizes VirtualList scrolling performance by reducing per-scroll work, primarily by scanning visible ranges from the current render window using cached offsets and updating padding offsets incrementally.
Changes:
- Update visible-range calculation to scan from
renderRange.startusingoffset.top, avoiding always walking from index 0. - Incrementally update
offset.top/offset.bottomwhen the render range shifts, falling back to full recompute when the item count changes. - Avoid
previousItemsarray cloning by trackingpreviousLengthand head/tail IDs, and add ajumpToIndexfast path when the target is already rendered.
Reduce per-scroll-frame work from O(totalItems) to O(visibleItems + delta)
- Start visible-range scanning from renderRange.start using cached
offset.top instead of always walking from index 0
- Limit DOM height reads to elements inside the current render range
- Replace redundant O(n) calculateHeightSum(0, renderRange.start) with
cached offset.top in compensateScrollForResize
- Update top/bottom offsets incrementally (O(|rangeDelta|)) when the
render range shifts, falling back to full recompute only when item
count changes
- Replace previousItems array clone with three scalars
(previousLength, previousHeadId, previousTailId) to detect
prepend/append/replace without O(n) copy
- Remove $state proxy from heightMap (never read in reactive context)
- Add jumpToIndex fast path: skip teardown when target is already in
render range, compute scrollTop as offset.top + partial sum
O(renderRange.size) instead of O(index)
- Cache repeated template expressions with {@const} in #each block
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Reduce per-scroll-frame work from O(items) to O(visibleItems + delta)
cached offset.top instead of always walking from index 0
the render range shifts, falling back to a full recompute only when
the item count changes
previousHeadId, previousTailId) to detect prepend/append/replace