When switching tables, the table-switch branch writes fresh data for the
new table, which re-fires the same callback with data_previous still
holding the old table's rows. find_changed_cell only checked row count
before diffing, so if the two tables happened to have the same number of
rows it would zip mismatched-schema dicts and report a spurious changed
cell (usually the PK column), producing a confusing red alert right after
switching tables.
Colin Maudry