Make statement lines readonly when origins are filled

Closes #14965
(grafted from ad46e99cf76bee15373625a6ea0ec4e57a58ff06)

--HG--
branch : 7.0

modules/account_statement/statement.py

@@ -101,7 +101,10 @@ class Statement(Workflow, ModelSQL, ModelView):
         states=_number_states)
     lines = fields.One2Many('account.statement.line', 'statement',
         'Lines', states={
-            'readonly': (Eval('state') != 'draft') | ~Eval('journal'),
+            'readonly': (
+                (Eval('state') != 'draft')
+                | ~Eval('journal')
+                | Eval('origins', [])),
             })
     origins = fields.One2Many('account.statement.origin', 'statement',
         "Origins", states={
@@ -263,11 +266,15 @@ class Statement(Workflow, ModelSQL, ModelView):
             origin.lines = lines
         self.origins = origins
 
-    @fields.depends('lines')
+    @fields.depends('lines', 'origins')
     def on_change_lines(self):
         pool = Pool()
         Line = pool.get('account.statement.line')
 
+        if self.origins:
+            # let origin manage the pending amounts
+            return
+
         invoices = {l.invoice for l in self.lines if l.invoice}
         invoice_id2amount_to_pay = {}
         for invoice in invoices: