ln: qt channel open: fix max channel size

electrum/gui/qt/channels_list.py

@@ -202,6 +202,12 @@ class ChannelsList(MyTreeView):
         remote_nodeid.setCursorPosition(0)
         if not d.exec_():
             return
-        funding_sat = '!' if max_button.isChecked() else amount_e.get_amount()
+        if max_button.isChecked() and amount_e.get_amount() < LN_MAX_FUNDING_SAT:
+            # if 'max' enabled and amount is strictly less than max allowed,
+            # that means we have fewer coins than max allowed, and hence we can
+            # spend all coins
+            funding_sat = '!'
+        else:
+            funding_sat = amount_e.get_amount()
         connect_str = str(remote_nodeid.text()).strip()
         self.parent.open_channel(connect_str, funding_sat, 0)

electrum/lnutil.py

@@ -27,7 +27,7 @@ if TYPE_CHECKING:
 HTLC_TIMEOUT_WEIGHT = 663
 HTLC_SUCCESS_WEIGHT = 703
 
-LN_MAX_FUNDING_SAT = pow(2, 24)
+LN_MAX_FUNDING_SAT = pow(2, 24) - 1
 
 # dummy address for fee estimation of funding tx
 def ln_dummy_address():