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.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 @@ def on_wt_url():
             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_BUDGET_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_BUDGET_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_BUDGET_FEE_MAX_MILLIONTHS)
+        except ValueError:
+            lnfee_spos = 0
+        lnfee_slider.setSliderPosition(lnfee_spos)
+        lnfee_vlabel = QLabel("")
+        lnfee_update_vlabel(self.config.LIGHTNING_PAYMENT_BUDGET_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 @@ def on_history_rates(checked):
         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 +1043,8 @@ def _default_swapserver_url(self) -> str:
     LIGHTNING_PAYMENT_BUDGET_FEE_MAX_MILLIONTHS = ConfigVar(
         'lightning_payment_budget_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."""),
     )
     LIGHTNING_PAYMENT_BUDGET_FEE_CUTOFF_MSAT = ConfigVar(
         'lightning_payment_budget_fee_cutoff_msat', default=10_000,  # 10 sat