Multi-Select: Box-Select: fixed an issue where items out of horizontal view would sometimes lead to incorrect merging of sequential selection requests. (#7994, #1861, #6518)

SetNextItemSelectionUserData() could use g.NextItemData.SelectionUserData if we could guarantee if would be valid when nesting multi-select blocks.
But it doesn't make much of a difference as whole ImGuiMultiSelectTempData fits in 2 cache lines.