qt settings: expose LIGHTNING_PAYMENT_BUDGET_FEE_MAX_MILLIONTHS

Expose as a slider; perhaps it is less footgunny this way?

electrum/gui/qt/settings_dialog.py

@@ -30,7 +30,7 @@ from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import (QComboBox,  QTabWidget, QDialog,
                              QSpinBox,  QFileDialog, QCheckBox, QLabel,
                              QVBoxLayout, QGridLayout, QLineEdit,
-                             QPushButton, QWidget, QHBoxLayout)
+                             QPushButton, QWidget, QHBoxLayout, QSlider)
 
 from electrum.i18n import _, languages
 from electrum import util, paymentrequest
@@ -155,6 +155,37 @@ class SettingsDialog(QDialog, QtEventListener):
             self.config.WATCHTOWER_CLIENT_URL = url
         self.watchtower_url_e.editingFinished.connect(on_wt_url)
 
+        lnfee_hlabel = HelpLabel.from_configvar(self.config.cv.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS)
+        lnfee_map = [500, 1_000, 3_000, 5_000, 10_000, 20_000, 30_000, 50_000]
+        def lnfee_update_vlabel(fee_val: int):
+            lnfee_vlabel.setText(_("{}% of payment").format(f"{fee_val / 10 ** 4:.2f}"))
+        def lnfee_slider_moved():
+            pos = lnfee_slider.sliderPosition()
+            fee_val = lnfee_map[pos]
+            lnfee_update_vlabel(fee_val)
+        def lnfee_slider_released():
+            pos = lnfee_slider.sliderPosition()
+            fee_val = lnfee_map[pos]
+            self.config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS = fee_val
+        lnfee_slider = QSlider(Qt.Horizontal)
+        lnfee_slider.setRange(0, len(lnfee_map)-1)
+        lnfee_slider.setTracking(True)
+        try:
+            lnfee_spos = lnfee_map.index(self.config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS)
+        except ValueError:
+            lnfee_spos = 0
+        lnfee_slider.setSliderPosition(lnfee_spos)
+        lnfee_vlabel = QLabel("")
+        lnfee_update_vlabel(self.config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS)
+        lnfee_slider.valueChanged.connect(lnfee_slider_moved)
+        lnfee_slider.sliderReleased.connect(lnfee_slider_released)
+        lnfee_hbox = QHBoxLayout()
+        lnfee_hbox.setContentsMargins(0, 0, 0, 0)
+        lnfee_hbox.addWidget(lnfee_vlabel)
+        lnfee_hbox.addWidget(lnfee_slider)
+        lnfee_hbox_w = QWidget()
+        lnfee_hbox_w.setLayout(lnfee_hbox)
+
         alias_label = HelpLabel.from_configvar(self.config.cv.OPENALIAS_ID)
         alias = self.config.OPENALIAS_ID
         self.alias_e = QLineEdit(alias)
@@ -351,6 +382,7 @@ class SettingsDialog(QDialog, QtEventListener):
         lightning_widgets.append((trampoline_cb, None))
         lightning_widgets.append((legacy_add_trampoline_cb, None))
         lightning_widgets.append((remote_wt_cb, self.watchtower_url_e))
+        lightning_widgets.append((lnfee_hlabel, lnfee_hbox_w))
         fiat_widgets = []
         fiat_widgets.append((QLabel(_('Fiat currency')), ccy_combo))
         fiat_widgets.append((QLabel(_('Source')), ex_combo))

electrum/simple_config.py

@@ -1043,7 +1043,10 @@ This will result in longer routes; it might increase your fees and decrease the
     LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS = ConfigVar(
         'lightning_payment_fee_max_millionths', default=10_000,  # 1%
         type_=int,
-        short_desc=lambda: _("Max lightning fees (%) to pay"),
+        short_desc=lambda: _("Max lightning fees to pay"),
+        long_desc=lambda: _("""When sending lightning payments, this value is an upper bound for the fees we allow paying, proportional to the payment amount. The fees are paid in addition to the payment amount, by the sender.
+
+Warning: setting this to too low will result in lots of payment failures."""),
     )
     LIGHTNING_PAYMENT_FEE_CUTOFF_MSAT = ConfigVar(
         'lightning_payment_fee_cutoff_msat', default=10_000,  # 10 sat