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import { createEffect, on, onCleanup } from "solid-js"
import { createStore } from "solid-js/store"
import { createResizeObserver } from "@solid-primitives/resize-observer"
export interface AutoScrollOptions {
working: () => boolean
onUserInteracted?: () => void
overflowAnchor?: "none" | "auto" | "dynamic"
bottomThreshold?: number
}
export function createAutoScroll(options: AutoScrollOptions) {
let scroll: HTMLElement | undefined
let settling = false
let settleTimer: ReturnType<typeof setTimeout> | undefined
let cleanup: (() => void) | undefined
let resizeFrame: number | undefined
const threshold = () => options.bottomThreshold ?? 10
const [store, setStore] = createStore({
contentRef: undefined as HTMLElement | undefined,
userScrolled: false,
})
const active = () => options.working() || settling
const distanceFromBottom = (el: HTMLElement) => {
return el.scrollHeight - el.clientHeight - el.scrollTop
}
const scrollToBottomNow = (behavior: ScrollBehavior) => {
const el = scroll
if (!el) return
el.scrollTo({ top: el.scrollHeight, behavior })
}
const scrollToBottom = (force: boolean) => {
if (!force && !active()) return
const el = scroll
if (!el) return
if (!force && store.userScrolled) return
if (force && store.userScrolled) setStore("userScrolled", false)
const distance = distanceFromBottom(el)
if (distance < 2) return
// For auto-following content we prefer immediate updates to avoid
// visible "catch up" animations while content is still settling.
scrollToBottomNow("auto")
}
const stop = () => {
if (store.userScrolled) return
setStore("userScrolled", true)
options.onUserInteracted?.()
}
const handleWheel = (e: WheelEvent) => {
if (e.deltaY >= 0) return
// If the user is scrolling within a nested scrollable region (tool output,
// code block, etc), don't treat it as leaving the "follow bottom" mode.
// Those regions opt in via `data-scrollable`.
const el = scroll
const target = e.target instanceof Element ? e.target : undefined
const nested = target?.closest("[data-scrollable]")
if (el && nested && nested !== el) return
stop()
}
const handleScroll = () => {
const el = scroll
if (!el) return
if (distanceFromBottom(el) < threshold()) {
if (store.userScrolled) setStore("userScrolled", false)
return
}
stop()
}
const handleInteraction = () => {
if (!active()) return
stop()
}
const updateOverflowAnchor = (el: HTMLElement) => {
const mode = options.overflowAnchor ?? "dynamic"
if (mode === "none") {
el.style.overflowAnchor = "none"
return
}
if (mode === "auto") {
el.style.overflowAnchor = "auto"
return
}
el.style.overflowAnchor = store.userScrolled ? "auto" : "none"
}
createResizeObserver(
() => store.contentRef,
() => {
if (!active()) return
if (store.userScrolled) return
if (resizeFrame !== undefined) return
resizeFrame = requestAnimationFrame(() => {
resizeFrame = undefined
scrollToBottom(false)
})
},
)
createEffect(
on(options.working, (working) => {
settling = false
if (settleTimer) clearTimeout(settleTimer)
settleTimer = undefined
if (working) {
if (!store.userScrolled) scrollToBottom(true)
return
}
settling = true
settleTimer = setTimeout(() => {
settling = false
}, 300)
}),
)
createEffect(() => {
// Track `userScrolled` even before `scrollRef` is attached, so we can
// update overflow anchoring once the element exists.
store.userScrolled
const el = scroll
if (!el) return
updateOverflowAnchor(el)
})
onCleanup(() => {
if (settleTimer) clearTimeout(settleTimer)
if (resizeFrame !== undefined) cancelAnimationFrame(resizeFrame)
if (cleanup) cleanup()
})
return {
scrollRef: (el: HTMLElement | undefined) => {
if (cleanup) {
cleanup()
cleanup = undefined
}
scroll = el
if (!el) return
updateOverflowAnchor(el)
el.addEventListener("wheel", handleWheel, { passive: true })
cleanup = () => {
el.removeEventListener("wheel", handleWheel)
}
},
contentRef: (el: HTMLElement | undefined) => setStore("contentRef", el),
handleScroll,
handleInteraction,
pause: stop,
resume: () => {
if (store.userScrolled) setStore("userScrolled", false)
scrollToBottom(true)
},
scrollToBottom: () => scrollToBottom(false),
forceScrollToBottom: () => scrollToBottom(true),
userScrolled: () => store.userScrolled,
}
}
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