rework pagination - fixes #491

previously, when adding items either at the beginnig (e.g. new notes
coming in while we're not looking at the top of the timeline) or a the
end (e.g. more items arriving from a background fetch) of a paginated
view, the resulting list got truncated to `displayLimit`, potentially
throwing data away and causing a new fetch.

This, coupled with the async nature of scrolling & fetching, could
cause weird results.

Also, `offset` was always incremented by the size of the fetched
results, even if not all of them were displayed, meant that it was
possible for offset-based pagination to drop items.

Finally, the "queue" of new items (usually, new notes) also got
truncated to `displayLimit`, which again could drop items (this effect
was usually masked by the first point: when scrolling to the top of
the timeline, if the queue's length was equal to `displayLimit`, those
notes displaced any existing ones, `unshiftItems` set
`more.value=true`, you got scrolled to the top, and notes were fetched
again, so you lost your position but at least all notes got shown,
eventually)

packages/frontend/src/components/MkPagination.vue

@@ -395,10 +395,10 @@ const prepend = (item: MisskeyEntity): void => {
  * @param newItems 新しいアイテムの配列
  */
 function unshiftItems(newItems: MisskeyEntity[]) {
-	const length = newItems.length + items.value.size;
-	items.value = new Map([...arrayToEntries(newItems), ...items.value].slice(0, props.displayLimit));
-
-	if (length >= props.displayLimit) more.value = true;
+	const prevLength = items.value.size;
+	items.value = new Map([...arrayToEntries(newItems), ...items.value].slice(0, newItems.length + props.displayLimit));
+	// if we truncated, mark that there are more values to fetch
+	if (items.value.size < prevLength) more.value = true;
 }
 
 /**
@@ -406,10 +406,10 @@ function unshiftItems(newItems: MisskeyEntity[]) {
  * @param oldItems 古いアイテムの配列
  */
 function concatItems(oldItems: MisskeyEntity[]) {
-	const length = oldItems.length + items.value.size;
-	items.value = new Map([...items.value, ...arrayToEntries(oldItems)].slice(0, props.displayLimit));
-
-	if (length >= props.displayLimit) more.value = true;
+	const prevLength = items.value.size;
+	items.value = new Map([...items.value, ...arrayToEntries(oldItems)].slice(0, oldItems.length + props.displayLimit));
+	// if we truncated, mark that there are more values to fetch
+	if (items.value.size < prevLength) more.value = true;
 }
 
 function executeQueue() {
@@ -418,7 +418,7 @@ function executeQueue() {
 }
 
 function prependQueue(newItem: MisskeyEntity) {
-	queue.value = new Map([[newItem.id, newItem], ...queue.value].slice(0, props.displayLimit) as [string, MisskeyEntity][]);
+	queue.value = new Map([[newItem.id, newItem], ...queue.value] as [string, MisskeyEntity][]);
 }
 
 /*