NodesInserted/NodesRemoved reported a multi-node change (a node plus its visible
descendants -- a contiguous run) as a sequence of single-item Add/Remove events,
but the underlying tree updates Count by the WHOLE run before they fire. A
consumer that reconciles the change one item at a time and re-indexes the source
mid-sequence then reads against a Count that has already dropped by the full run:
Avalonia's SelectionModel does exactly this (it re-reads SelectedItems while
handling each Remove), indexing past the end and throwing ArgumentOutOfRangeException
out of TreeFlattener's indexer whenever a selection was live during a bulk reshape
(e.g. toggling UseNestedNamespaceNodes). It only surfaced under full-suite timing,
which is why it read as flaky.
Raise one ranged event for the whole run instead, so the notification matches the
tree's state in a single step and Count/indices stay in agreement. The indexer
keeps throwing on genuine out-of-range access. Avalonia 12.4's virtualization and
selection handle ranged Add/Remove (every expand/collapse exercises it); full
headless suite is green.
Siegfried Pammer
4585be8de9