Why change the assignment of unusable rows for advice columns?

[cx19981114]

Now in the advice columns, the blinding factors assignment to unusable rows is changed to a value of Scalar::one for the last row

In my understanding, the blinding factors are added to ensure zero knowledge. Is it possible to modify the deterministic assignment to the last line to ensure zero knowledge?or is there some other purpose?

In Zcash
https://github.com/zcash/halo2/blob/76b3f892a9d598923bbb5a747701fff44ae4c0ea/halo2_proofs/src/plonk/prover.rs#L294-L298

In Scroll-tech

halo2/halo2_proofs/src/plonk/prover.rs

Lines 446 to 454 in 9bf3562

	// Add blinding factors to advice columns
	for advice_values in &mut advice_values {
	//for cell in &mut advice_values[unusable_rows_start..] {
	//*cell = C::Scalar::random(&mut rng);
	//*cell = C::Scalar::one();
	//}
	let idx = advice_values.len() - 1;
	advice_values[idx] = Scheme::Scalar::one();
	}

modify in this commit 4b53eee

[kunxian-xia]

In the scenario of zkEVM, we do not rely too much on the zk property. Setting blinding factors to zeros can making the proof more deterministic, therefore make it more easier to debug the cpu prover and gpu prover (when there are inconsistency between them).