Minting to the active bin returns reduced/unfair shares for LPs

When minting liquidity for an active bin, the shares minted for the liquidity provider are overpriced, further disincentivizing liquidity provision to the active bins and causing unfair share distribution for the providers.

/// Handle composition fees for active bin
#[storage(read, write)]
fn handle_composition_fees(
    storage_keys: MintStorageKeys,
    internal_bin_id: InternalBinId,
    bin_reserves: Amounts,
    amounts_in: Amounts,
    fees: Amounts,
    price: Price,
    total_shares_in_bin: u256,
) -> (u256, Amounts) {
    let amounts_after_fees = amounts_in.sub(fees);
    let user_liquidity = get_liquidity(amounts_after_fees, price);
    // ... code snippet ..

    let non_protocol_fees = fees.sub(protocol_fees);
    let bin_reserves_with_fees = bin_reserves.add(non_protocol_fees);
    let bin_liquidity = get_liquidity(bin_reserves_with_fees, price);

    let shares = math::uint256x256_math::mul_div_round_down(user_liquidity,
total_shares_in_bin, bin_liquidity);
    (shares, amounts_in)
}

bin_reserves is topped up with the non-protocol fees, and the calculation for the shares uses this overestimated liquidity, not taking into account that total fees have already been deducted from the user.

While the code for calculating user obtained shares diminishes the LP value, the code follows strictly the LFJ V2 formula which has been accepted by the market for a long time already. That is why the chosen severity for it is Medium.

Liquidity providers minting to active bins receive fewer shares than the value they contribute, materially diminishing LP value over time and disincentivizing active-bin liquidity provision.

Use the normal bin_reserves for the calculation of the shares the user gets when providing liquidity for an active bin.

Acknowledged. The team chose to retain the LFJ V2 formula behaviour for compatibility.