Function table_block_at_with_fences 

pub fn table_block_at_with_fences(
    buffer: &EditorBuffer,
    row: usize,
    fences: &[usize],
) -> Option<TableBlock>

table_block_at with a caller-provided fence index.

fences is the sorted output of fence_rows; the fenced-region check is O(log F). Remaining work per call is bounded (upward walk budget 512, body extension), so this is safe per visible row per frame as long as fences is computed once per document version.