ancestorIsLastMask property
Bitmask of "is this ancestor the last child?" over ancestor depth levels.
Bit d (for d in 0 .. depth-1) is set when the ancestor at depth d
is the last child of its parent — meaning no vertical continuation line is
drawn at that depth. Replaces a per-node List<bool> to avoid a heap
allocation for every flattened node (the flags are read only when a row is
painted, i.e. only for visible rows). Tree depth is therefore capped at
maxDepth.
Implementation
final int ancestorIsLastMask;