Gets the widths of the blocks.

        Note: This works with the property structure `_widths_cache` to avoid
            having to recompute these values each time they are needed.

def block_widths(self):
        """Gets the widths of the blocks.

        Note: This works with the property structure `_widths_cache` to avoid
            having to recompute these values each time they are needed.
        """
        if self._widths_cache is None:
            # The first column will have the correct lengths. We have an
            # invariant that requires that all blocks be the same width in a
            # column of blocks.
            self._widths_cache = (
                [obj.width() for obj in self._partitions_cache[0]]
                if len(self._partitions_cache) > 0
                else []
            )
        return self._widths_cache