Function computeVirtualScrollColdStartRange

src/hooks/useVirtualScroll.ts:147–197

({
  itemKeys,
  heightCache,
  getWarmHeight,
  estimateItemHeight,
  previousViewportHeight,
  terminalRows = 0,
  coldStartCount = COLD_START_COUNT,
  overscanRows = OVERSCAN_ROWS,
}: VirtualScrollColdStartRangeInput)

}

export function computeVirtualScrollColdStartRange({
  itemKeys,
  heightCache,
  getWarmHeight,
  estimateItemHeight,
  previousViewportHeight,
  terminalRows = 0,
  coldStartCount = COLD_START_COUNT,
  overscanRows = OVERSCAN_ROWS,
}: VirtualScrollColdStartRangeInput): readonly [number, number] {
  coldStartCount = resolveVirtualScrollColdStartCount({
    previousViewportHeight,
    terminalRows,
    coldStartCount,
  })
  const end = itemKeys.length
  const fallbackStart = Math.max(0, end - coldStartCount)
  const coldStartViewport =
    previousViewportHeight > 0 ? previousViewportHeight : terminalRows

  if (end === 0 || coldStartViewport <= 0) {
    return [fallbackStart, end]
  }

  // Transcript entry is the long-history hot path: prompt mode already had a
  // real viewport and often measured the tail rows we are about to show.
  // Reuse those measured tail heights during the transient viewportHeight=0 /
  // scrollTop<0 pre-layout render so we do not over-mount the fixed 30-row
  // cold-start window for giant file/code blocks. If measured/warm heights are
  // unavailable, allow a cheap render-time estimate for the tail item. This
  // keeps the first transcript-entry mount row-budgeted instead of
  // fixed-message-count for giant fenced-code messages, but still falls back
  // to the old COLD_START_COUNT behavior when the tail shape is unknown.
  const budget = coldStartViewport + overscanRows
  let start = end
  let coverage = 0
  while (start > fallbackStart && coverage < budget) {
    const key = itemKeys[start - 1]!
    const height =
      heightCache.get(key) ??
      getWarmHeight?.(key) ??
      estimateItemHeight?.(start - 1)
    if (height === undefined) {
      return [fallbackStart, end]
    }
    coverage += Math.max(1, height)
    start--
  }

  return coverage >= budget ? [start, end] : [fallbackStart, end]
}

export type VirtualScrollOffsetsState = {
  arr: Float64Array