qt receive tab: "receive address" is now coloured red if already used

closes #3812
closes #5374

electrum/gui/qt/main_window.py

@@ -907,6 +907,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         msg = _('Bitcoin address where the payment should be received. Note that each payment request uses a different Bitcoin address.')
         self.receive_address_label = HelpLabel(_('Receiving address'), msg)
         self.receive_address_e.textChanged.connect(self.update_receive_qr)
+        self.receive_address_e.textChanged.connect(self.update_receive_address_styling)
         self.receive_address_e.setFocusPolicy(Qt.ClickFocus)
         grid.addWidget(self.receive_address_label, 0, 0)
         grid.addWidget(self.receive_address_e, 0, 1, 1, -1)
@@ -1152,6 +1153,16 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         if self.qr_window and self.qr_window.isVisible():
             self.qr_window.qrw.setData(uri)
 
+    def update_receive_address_styling(self):
+        addr = str(self.receive_address_e.text())
+        if self.wallet.is_used(addr):
+            self.receive_address_e.setStyleSheet(ColorScheme.RED.as_stylesheet(True))
+            self.receive_address_e.setToolTip(_("This address has already been used. "
+                                                "For better privacy, do not reuse it for new payments."))
+        else:
+            self.receive_address_e.setStyleSheet("")
+            self.receive_address_e.setToolTip("")
+
     def set_feerounding_text(self, num_satoshis_added):
         self.feerounding_text = (_('Additional {} satoshis are going to be added.')
                                  .format(num_satoshis_added))

electrum/gui/qt/request_list.py

@@ -105,9 +105,10 @@ class RequestList(MyTreeView):
         except InternalAddressCorruption as e:
             self.parent.show_error(str(e))
             addr = ''
-        if not current_address in domain and addr:
+        if current_address not in domain and addr:
             self.parent.set_receive_address(addr)
         self.parent.new_request_button.setEnabled(addr != current_address)
+        self.parent.update_receive_address_styling()
 
         self.model().clear()
         self.update_headers(self.__class__.headers)