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diff --git a/src/composables/useVirtualScrolling.js b/src/composables/useVirtualScrolling.js
index 9beb8c591b..fa6a3de030 100644
--- a/src/composables/useVirtualScrolling.js
+++ b/src/composables/useVirtualScrolling.js
@@ -1,406 +1,411 @@
import { first, last } from 'lodash-es'
import { computed, nextTick, ref, toValue, watch } from 'vue'
export function useVirtualScrolling({
// For debugging
name = 'Generic',
// Master toggle
enabled,
// List of items
list,
// Container of items, used for measuring scroll position
body,
// vertical offset to account for fixed and sticky headers
offset,
// useScrollPosition composable, used to prevent dupicating instances
scrollPositionInstance,
// buffer zone, the amount of placeholder heights to include
buffer,
// whether to use scroll compensation when elements above anchor change
scrollCompensation,
// How to handle collapse/expansion (going from 0 elements to full and back)
// - false - don't do scroll compensation at all
// - 'height' - compensate scroll according to list's height
// - 'item' - same as height but uses anchor element's top offset
// instead of whole height
collapseMode,
// Placeholder height specification. Must be a function.
// function will be called either:
// - without arguments (for generic placeholder, i.e. buffer zone size)
// - with id (for specific item placeholders)
// function must return ref pointing to height
getPlaceholderHeight,
// Invert grow direction. Useful for chat views. Makes scroll compensation
// work in reverse - adding items to bottom scrolls down, adding items to
// top doesn't scroll up.
invertDirection = false,
}) {
// # Suspension
const unsuspendibleIds = ref(new Set())
const changeSuspendState = ({ id, suspendable }) => {
if (suspendable) {
unsuspendibleIds.value.delete(id)
} else {
unsuspendibleIds.value.add(id)
}
}
// # Heights mapping.
const heights = ref(new Map())
const heightChart = computed(() => {
// Map every height and suspendable state
const chart = list.value.map((item) => {
const { id } = item
const height = (() => {
if (heights.value.has(id)) {
return heights.value.get(id)
} else {
return getPlaceholderHeight(id).value
}
})()
const suspendable = !unsuspendibleIds.value.has(id)
const real = heights.value.has(id)
return { id, height, suspendable, real, visible: true }
})
// Walk over the list to set top offsets
chart.reduce((sum, item) => {
item.top = sum
return sum + item.height
}, 0)
return chart
})
const updateVirtualHeight = ({ id, height }) => {
heights.value.set(id, height)
}
const {
top: scrollY,
scrollBy,
vHeight,
hasReachedTop,
hasReachedBottom,
} = scrollPositionInstance
// Real scroll boundary, relative to body's bounds
const topScrollBoundary = ref(0)
const bottomScrollBoundary = ref(0)
const updateBoundaries = () => {
if (!toValue(enabled)) return
if (!body.value) return // Not mounted yet
const { top } = body.value.getBoundingClientRect()
const distanceItemTopToWindowTop = 0 - top + (toValue(offset) ?? 0)
const distanceItemTopToWindowBottom = vHeight.value - top
// Technically, bottom scroll boundary should be distance
// from element's top border to window's bottom border,
// but it just so happens that it is equal to this.
// You can verify it by drawing the boxes and measuring
// distances yourself, I know I did. Geometry, man...
topScrollBoundary.value = distanceItemTopToWindowTop
bottomScrollBoundary.value = distanceItemTopToWindowBottom
}
const windowWatcher = watch(vHeight, updateBoundaries)
const scrollWatcher = watch(scrollY, updateBoundaries)
const heightWatcher = watch(heightChart, updateBoundaries)
const bodyWatcher = watch(body, updateBoundaries)
watch(vHeight, (neu, old) => {
if (toValue(invertDirection)) {
// Only compensate (stick to bottom) when shrinking
scrollBy(0, Math.max(old - neu, 0))
// compensating in other direction causes overscrolling
}
})
const pauseWatchers = () => {
windowWatcher.pause()
scrollWatcher.pause()
heightWatcher.pause()
bodyWatcher.pause()
}
const resumeWatchers = (skipUpdate = false) => {
windowWatcher.resume()
scrollWatcher.resume()
heightWatcher.resume()
bodyWatcher.resume()
if (skipUpdate) return
updateBoundaries()
}
watch(enabled, (val) => {
if (val) {
resumeWatchers()
} else {
pauseWatchers()
}
})
// # Visiblity
// Add buffer zone to boundary, equal to approx 3 items heights
const bufferZone = computed(
() => getPlaceholderHeight().value * (toValue(buffer) ?? 3),
)
const checkVisible = ({ top, height }) => {
const itemTopBoundary = top
const itemBottomBoundary = top + height
// Include buffer zone
const finalTopScrollBoundary = topScrollBoundary.value - bufferZone.value
const finalBottomScrollBoundary =
bottomScrollBoundary.value + bufferZone.value
// To be visible, item's bottom boundary shoud be below top scroll boundary)
const isBelowTopBoundary = itemBottomBoundary > finalTopScrollBoundary
// To be visible, item's top boundary shoud be above bottom scroll boundary)
const isAboveBottomBoundary = itemTopBoundary < finalBottomScrollBoundary
// This accounts for the case where item's boundaries exceed scroll boundary
return isBelowTopBoundary && isAboveBottomBoundary
}
const heightChartVisibility = computed(() =>
heightChart.value.map((heightChartItem) => ({
...heightChartItem,
visible: checkVisible(heightChartItem),
})),
)
const heightChartGrouped = computed(() => {
const chart = enabled.value ? heightChartVisibility : heightChart
// Group invisible items into spacers
return chart.value.reduce((acc, heightChartItem, index) => {
const { suspendable, visible, height, top, id } = heightChartItem
const present = visible || !suspendable
if (present) {
return [...acc, { type: 'item', height, top, id }]
} else {
// Reusing previous item if possible
const previousItem = acc[acc.length - 1]
const usingPreviousItem = previousItem?.type === 'spacer'
// We only really care for height and id of spacer, everything else
// is just for debugging
const spacer = usingPreviousItem
? previousItem
: {
type: 'spacer',
top: Number.POSITIVE_INFINITY,
height: 0,
id: `i${index}`, // used for v-for key attribute
ids: new Set(),
}
spacer.ids.add(id)
spacer.height += height
if (top < spacer.top) spacer.top = top
// If we used previous item there is no need to push it to array
if (usingPreviousItem) {
return acc
} else {
return [...acc, spacer]
}
}
}, [])
})
// ## Scroll compensation
watch(
heightChart,
async (newVal, oldVal) => {
if (newVal.length === 0 && oldVal.length === 0) return 0
const explosion = oldVal.length === 0 && newVal.length !== 0
const implosion = oldVal.length !== 0 && newVal.length === 0
const compensation = toValue(scrollCompensation)
const diff = (() => {
if (explosion) {
if (!compensation) return 0
if (toValue(collapseMode) === 'height' || toValue(invertDirection)) {
const newBottomElement = last(newVal)
return newBottomElement.top + newBottomElement.height
} else {
return 0
}
} else if (implosion) {
if (!compensation) return 0
const oldBottomElement = last(oldVal)
return 0 - oldBottomElement.top - oldBottomElement.height
} else {
const contextChange = (() => {
// HACK ignore date separators and fake messages in chat view
const oldIds = new Set(
oldVal
.map(({ id }) => id)
.filter((id) => !id.startsWith('fake-')),
)
const newIds = new Set(
newVal
.map(({ id }) => id)
.filter((id) => !id.startsWith('fake-')),
)
if (oldVal.length <= newVal.length) {
return [...oldIds].some((id) => !newIds.has(id))
} else {
return [...newIds].some((id) => !oldIds.has(id))
}
})()
if (contextChange) {
return 0
}
/* Ok, here's a thing. Both Timeline and Conversation have virtual scrolling.
* But since Conversation can be inside Timeline (in fact it's chock-full of
* them) it creates a problem. Virtual scrolling involves a lot of scroll
* compensation. Elements appearing, disappearing, changing sizes etc. What
* happens if a single conversation gets expanded? Conversation sees influx
* of new posts and compensates for them appearing above to keep old items
* in screen. Next, timeline sees that Conversation changed height and ALSO
* compensates for that changed height. We get double the compensation and
* result ends up being all wrong, we scrolled way past the conversation.
*
* Easy, just don't compensate in Timeline, right? Wrong. We still have two
* cases where we do want compensation in Timeline - all the cases that
* aren't handled by Conversation. Namely:
* - Change in amount of Conversations
* - Change in height of freshly-rendered Conversations.
*
* First one is simple. We scrolled down and made a new post - it appeared
* at the top but we don't want screen to scroll.
*
* Second one is tricky. Here's a real situation:
* 1. We scroll down in Timeline.
* 2. Open a thread (navigate to it, not expand!)
* 3. Press "back".
* Now we have a situation: vue-router restored our scroll position but
* since Timeline was removed and re-created its heightChart is full of
* fake placeholders. This naturally means our actual scroll position is
* all wrong. TODO: Store heightchart in timelines store?
* But it gets worse - when you start scrolling up elements begin getting
* their real heights back, and since there's no compensation timeline gets
* all jumpy! We can somewhat live with wrong scroll position but timeline
* being jumpy is unacceptable.
*
+ * TODO: Probably a good way to avoid it all is just to:
+ * - Don't do scroll compensation when nested
+ * - For expanded conversations use animated priority scrollTo
+ * something to do for Virtual Scrolling 2.2
+ *
* So, basically - if amount of elements changes, or amount of REAL elements
* changes - compensate anyway even if compensation is disabled.
*/
const isSticking = toValue(invertDirection)
? hasReachedBottom.value
: hasReachedTop.value
const compensateBecauseLengthChange =
!isSticking && newVal.length !== oldVal.length
const newReal = newVal.filter(({ real }) => real).length
const oldReal = oldVal.filter(({ real }) => real).length
const compensateBecauseReal =
oldReal > 0 && newReal > 0 && oldReal !== newReal
if (
!(
compensation ||
compensateBecauseReal ||
compensateBecauseLengthChange
)
) {
return 0
}
// Section is used for debugging only
const getShift = (section, a, b, find, getter) => {
const aItem = find(a)
if (!aItem) return 0
const bItem = b.find(({ id }) => id === aItem.id)
if (!bItem) return 0
return getter(bItem) - getter(aItem)
}
const expansion = (() => {
if (newVal.length < oldVal.length) return 0
return getShift(
'expansion',
oldVal.filter((item) => checkVisible(item)),
newVal,
(list) =>
list.find(
({ top, height }) => top + height < topScrollBoundary.value,
) ?? first(list),
({ top, height }) => top + height,
)
})()
const inverseCompensation = (() => {
if (!toValue(invertDirection)) return 0
if (newVal.length < oldVal.length) return 0
const shift = getShift(
'inverse',
oldVal.filter((item) => checkVisible(item)),
newVal,
(list) =>
list.findLast(({ top }) => top > bottomScrollBoundary.value) ??
last(list),
({ top }) => top,
)
if (isSticking) {
// Don't do 99999 here, it breaks virtual scrolling
const { height } = body.value.getBoundingClientRect()
return height // keep sticking
} else {
return shift
}
})()
const collapsing = (() => {
if (newVal.length >= oldVal.length) return 0
return -getShift(
'collapse',
newVal,
oldVal,
(list) => first(list),
({ top, height }) => top + height,
)
})()
return (
(toValue(invertDirection) ? inverseCompensation : expansion) +
collapsing
)
}
})()
if (diff !== 0) {
// Scroll by amount offset changed to keep it in view
topScrollBoundary.value += diff
bottomScrollBoundary.value += diff
await nextTick()
await scrollBy(0, diff)
}
resumeWatchers()
},
{ flush: 'post' },
)
// Misc
const reset = () => {
unsuspendibleIds.value = new Set()
heights.value = new Map()
}
return {
heightChart: heightChartGrouped,
changeSuspendState,
updateVirtualHeight,
pauseWatchers,
resumeWatchers,
updateBoundaries,
reset,
}
}

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