gui: fix coin control input size accounting for taproot spends

Author: theStack

src/qt/coincontroldialog.cpp

@@ -421,7 +421,19 @@ void CoinControlDialog::updateLabels(CCoinControl& m_coin_control, WalletModel *
         std::vector<unsigned char> witnessprogram;
         if (out.txout.scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram))
         {
-            nBytesInputs += (32 + 4 + 1 + (107 / WITNESS_SCALE_FACTOR) + 4);
+            // add input skeleton bytes (outpoint, scriptSig size, nSequence)
+            nBytesInputs += (32 + 4 + 1 + 4);
+
+            if (witnessversion == 0) { // P2WPKH
+                // 1 WU (witness item count) + 72 WU (ECDSA signature with len byte) + 34 WU (pubkey with len byte)
+                nBytesInputs += 107 / WITNESS_SCALE_FACTOR;
+            } else if (witnessversion == 1) { // P2TR key-path spend
+                // 1 WU (witness item count) + 65 WU (Schnorr signature with len byte)
+                nBytesInputs += 66 / WITNESS_SCALE_FACTOR;
+            } else {
+                // not supported, should be unreachable
+                throw std::runtime_error("Trying to spend future segwit version script");
+            }
             fWitness = true;
         }
         else if(ExtractDestination(out.txout.scriptPubKey, address))