add-txMaxFee-to-tbtcAmount

The Threshold DAO has decided to set the deposit transaction maximum fee to zero. Instead of modifying the deposit parameters, we adjust the amount of TBTC transferred to the L2 deposit owner by adding back the fee amount.

This change is implemented in the contracts that inherit from AbstractTBTCDepositor, not in the abstract contract itself. Specifically, the implementation adds depositTxMaxFee to the tbtcAmount value returned by _finalizeDeposit.

solidity/contracts/l2/L1BitcoinDepositor.sol

@@ -461,6 +461,16 @@ contract L1BitcoinDepositor is
             bytes32 l2DepositOwner
         ) = _finalizeDeposit(depositKey);
 
+        // The deposit transaction max fee is in the 1e8 satoshi precision.
+        // We need to convert them to the 1e18 TBTC precision.
+        (, , uint64 depositTxMaxFee, ) = bridge.depositParameters();
+        uint256 txMaxFee = depositTxMaxFee * SATOSHI_MULTIPLIER;
+
+        // The Threshold DAO decided the deposit transaction max fee to be 0.
+        // Instead of changing the deposit parameters, we add the fee to the
+        // amount of TBTC that is transferred to the L2 deposit owner.
+        tbtcAmount += txMaxFee;
+
         // slither-disable-next-line reentrancy-events
         emit DepositFinalized(
             depositKey,