qt/kivy: show warning when sending tx with high fee/amount ratio

related: #6162

electrum/gui/kivy/uix/screens.py

@@ -33,6 +33,7 @@ from electrum.util import (parse_URI, InvalidBitcoinURI, PR_PAID, PR_UNKNOWN, PR
 from electrum.plugin import run_hook
 from electrum.wallet import InternalAddressCorruption
 from electrum import simple_config
+from electrum.simple_config import FEERATE_WARNING_HIGH_FEE, FEE_RATIO_HIGH_WARNING
 from electrum.lnaddr import lndecode, parse_lightning_invoice
 from electrum.lnutil import RECEIVED, SENT, PaymentFailure
 
@@ -371,9 +372,14 @@ class SendScreen(CScreen):
             x_fee_address, x_fee_amount = x_fee
             msg.append(_("Additional fees") + ": " + self.app.format_amount_and_units(x_fee_amount))
 
-        feerate_warning = simple_config.FEERATE_WARNING_HIGH_FEE
-        if fee > feerate_warning * tx.estimated_size() / 1000:
-            msg.append(_('Warning') + ': ' + _("The fee for this transaction seems unusually high."))
+        feerate = Decimal(fee) / tx.estimated_size()  # sat/byte
+        fee_ratio = Decimal(fee) / amount if amount else 1
+        if fee_ratio >= FEE_RATIO_HIGH_WARNING:
+            msg.append(_('Warning') + ': ' + _("The fee for this transaction seems unusually high.")
+                       + f' ({fee_ratio*100:.2f}% of amount)')
+        elif feerate > FEERATE_WARNING_HIGH_FEE / 1000:
+            msg.append(_('Warning') + ': ' + _("The fee for this transaction seems unusually high.")
+                       + f' (feerate: {feerate:.2f} sat/byte)')
         self.app.protected('\n'.join(msg), self.send_tx, (tx,))
 
     def send_tx(self, tx, password):

electrum/gui/qt/confirm_tx_dialog.py

@@ -23,6 +23,7 @@
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+from decimal import Decimal
 from typing import TYPE_CHECKING, Optional, Union
 
 from PyQt5.QtWidgets import  QVBoxLayout, QLabel, QGridLayout, QPushButton, QLineEdit
@@ -31,7 +32,7 @@ from electrum.i18n import _
 from electrum.util import NotEnoughFunds, NoDynamicFeeEstimates
 from electrum.plugin import run_hook
 from electrum.transaction import Transaction, PartialTransaction
-from electrum.simple_config import FEERATE_WARNING_HIGH_FEE
+from electrum.simple_config import FEERATE_WARNING_HIGH_FEE, FEE_RATIO_HIGH_WARNING
 from electrum.wallet import InternalAddressCorruption
 
 from .util import (WindowModalDialog, ColorScheme, HelpLabel, Buttons, CancelButton,
@@ -240,17 +241,22 @@ class ConfirmTxDialog(TxEditor, WindowModalDialog):
             self.extra_fee_value.setVisible(True)
             self.extra_fee_value.setText(self.main_window.format_amount_and_units(x_fee_amount))
 
-        feerate_warning = FEERATE_WARNING_HIGH_FEE
-        low_fee = fee < self.wallet.relayfee() * tx.estimated_size() / 1000
-        high_fee = fee > feerate_warning * tx.estimated_size() / 1000
-        if low_fee:
+        amount = tx.output_value() if self.output_value == '!' else self.output_value
+        feerate = Decimal(fee) / tx.estimated_size()  # sat/byte
+        fee_ratio = Decimal(fee) / amount if amount else 1
+        if feerate < self.wallet.relayfee() / 1000:
             msg = '\n'.join([
                 _("This transaction requires a higher fee, or it will not be propagated by your current server"),
                 _("Try to raise your transaction fee, or use a server with a lower relay fee.")
             ])
             self.toggle_send_button(False, message=msg)
-        elif high_fee:
+        elif fee_ratio >= FEE_RATIO_HIGH_WARNING:
             self.toggle_send_button(True,
-                                    message=_('Warning') + ': ' + _("The fee for this transaction seems unusually high."))
+                                    message=_('Warning') + ': ' + _("The fee for this transaction seems unusually high.")
+                                            + f'\n({fee_ratio*100:.2f}% of amount)')
+        elif feerate > FEERATE_WARNING_HIGH_FEE / 1000:
+            self.toggle_send_button(True,
+                                    message=_('Warning') + ': ' + _("The fee for this transaction seems unusually high.")
+                                            + f'\n(feerate: {feerate:.2f} sat/byte)')
         else:
             self.toggle_send_button(True)

electrum/simple_config.py

@@ -33,6 +33,8 @@ FEERATE_STATIC_VALUES = [1000, 2000, 5000, 10000, 20000, 30000,
                          50000, 70000, 100000, 150000, 200000, 300000]
 FEERATE_REGTEST_HARDCODED = 180000  # for eclair compat
 
+FEE_RATIO_HIGH_WARNING = 0.05  # warn user if fee/amount for on-chain tx is higher than this
+
 
 _logger = get_logger(__name__)