moveCompletionSelection method
Moves the active completion by delta, wrapping at either end.
Implementation
bool moveCompletionSelection(int delta) {
if (_completionItems.isEmpty || delta == 0) return false;
_completionIndex = (_completionIndex + delta) % _completionItems.length;
if (_completionIndex < 0) _completionIndex += _completionItems.length;
return true;
}