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fix(useScroll): account for rounding errors when calculating arrivedState #2051

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merged 1 commit into from Aug 6, 2022
Merged

fix(useScroll): account for rounding errors when calculating arrivedState #2051

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12 changes: 10 additions & 2 deletions packages/core/useScroll/index.ts
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Expand Up @@ -50,6 +50,14 @@ export interface UseScrollOptions {
eventListenerOptions?: boolean | AddEventListenerOptions
}

/**
* We have to check if the scroll amount is close enough to some threshold in order to
* more accurately calculate arrivedState. This is because scrollTop/scrollLeft are non-rounded
* numbers, while scrollHeight/scrollWidth and clientHeight/clientWidth are rounded.
* https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollHeight#determine_if_an_element_has_been_totally_scrolled
*/
const ARRIVED_STATE_THRESHOLD_PIXELS = 1

/**
* Reactive scroll.
*
Expand Down Expand Up @@ -114,7 +122,7 @@ export function useScroll(
directions.right = scrollLeft > x.value
arrivedState.left = scrollLeft <= 0 + (offset.left || 0)
arrivedState.right
= scrollLeft + eventTarget.clientWidth >= eventTarget.scrollWidth - (offset.right || 0)
= scrollLeft + eventTarget.clientWidth >= eventTarget.scrollWidth - (offset.right || 0) - ARRIVED_STATE_THRESHOLD_PIXELS
x.value = scrollLeft

let scrollTop = eventTarget.scrollTop
Expand All @@ -127,7 +135,7 @@ export function useScroll(
directions.bottom = scrollTop > y.value
arrivedState.top = scrollTop <= 0 + (offset.top || 0)
arrivedState.bottom
= scrollTop + eventTarget.clientHeight >= eventTarget.scrollHeight - (offset.bottom || 0)
= scrollTop + eventTarget.clientHeight >= eventTarget.scrollHeight - (offset.bottom || 0) - ARRIVED_STATE_THRESHOLD_PIXELS
y.value = scrollTop

isScrolling.value = true
Expand Down