diff --git a/odoo/addons/test_new_api/tests/test_ui.py b/odoo/addons/test_new_api/tests/test_ui.py
index 7291e2b7c3e..5afc2876f4f 100644
--- a/odoo/addons/test_new_api/tests/test_ui.py
+++ b/odoo/addons/test_new_api/tests/test_ui.py
@@ -5,6 +5,13 @@ import odoo.tests
 class TestUi(odoo.tests.HttpCase):
 
     def test_01_admin_widget_x2many(self):
+        # FIXME: breaks if too many children of base.menu_tests
+
+        # This tour turns out to be quite sensible to the number of items in
+        # the base.menu_tests: it's specifically sequenced to be lower (after)
+        # the default, but doesn't account for the fact that it could
+        # "fall off" into the "o_extra_menu_items" section if the window is
+        # too small or there are too many items preceding it in the tests menu
         self.phantom_js("/web#action=test_new_api.action_discussions",
                         "odoo.__DEBUG__.services['web_tour.tour'].run('widget_x2many', 100)",
                         "odoo.__DEBUG__.services['web_tour.tour'].tours.widget_x2many.ready",
diff --git a/odoo/addons/test_testing_utilities/ir.model.access.csv b/odoo/addons/test_testing_utilities/ir.model.access.csv
index 697a7334581..c069b568ad4 100644
--- a/odoo/addons/test_testing_utilities/ir.model.access.csv
+++ b/odoo/addons/test_testing_utilities/ir.model.access.csv
@@ -14,3 +14,5 @@ access_test_testing_utilities_sub2,access_test_testing_utilities_sub2,model_test
 access_test_testing_utilities_sub3,access_test_testing_utilities_sub3,model_test_testing_utilities_sub3,,1,0,0,0
 access_test_testing_utilities_onchange_parent,access_test_testing_utilities_onchange_parent,model_test_testing_utilities_onchange_parent,,1,0,0,0
 access_test_testing_utilities_onchange_line,access_test_testing_utilities_onchange_line,model_test_testing_utilities_onchange_line,,1,0,0,0
+access_model_test_testing_utilities_onchange_count,access_model_test_testing_utilities_onchange_count,model_test_testing_utilities_onchange_count,,1,1,1,1
+access_test_testing_utilities_onchange_count_sub,access_test_testing_utilities_onchange_count_sub,model_test_testing_utilities_onchange_count_sub,,1,1,1,1
diff --git a/odoo/addons/test_testing_utilities/menu.xml b/odoo/addons/test_testing_utilities/menu.xml
index 3ca18ef2a50..fa5f4f37163 100644
--- a/odoo/addons/test_testing_utilities/menu.xml
+++ b/odoo/addons/test_testing_utilities/menu.xml
@@ -1,4 +1,6 @@
 <odoo>
+    <menuitem id="menu_ssf" parent="base.menu_tests" name="Server-side form"/>
+
     <record id="action_test_o2m_onchange" model="ir.actions.act_window">
         <field name="name">Test o2m onchange thing</field>
         <field name="type">ir.actions.act_window</field>
@@ -6,7 +8,7 @@
         <field name="view_mode">tree,form</field>
     </record>
 
-    <menuitem id="menu_o2m" parent="base.menu_tests"
+    <menuitem id="menu_o2m" parent="menu_ssf"
               name="O2M onchange test thing"
               action="action_test_o2m_onchange"/>
 
@@ -17,10 +19,21 @@
         <field name="view_mode">tree,form</field>
     </record>
 
-    <menuitem id="menu_m2o" parent="base.menu_tests"
+    <menuitem id="menu_m2o" parent="menu_ssf"
               name="test_default_and_onchange"
               action="action_test_m2o_onchange"/>
 
+    <record id="action_test_o2m_count" model="ir.actions.act_window">
+        <field name="name">test_change_number_of_lines</field>
+        <field name="type">ir.actions.act_window</field>
+        <field name="res_model">test_testing_utilities.onchange_count</field>
+        <field name="view_mode">tree,form</field>
+    </record>
+
+    <menuitem id="menu_o2m_count" parent="menu_ssf"
+              name="test_change_number_of_lines"
+              action="action_test_o2m_count"/>
+
     <record id="o2m_parent" model="ir.ui.view">
         <field name="name">Parent External Form</field>
         <field name="priority">1</field>
diff --git a/odoo/addons/test_testing_utilities/models.py b/odoo/addons/test_testing_utilities/models.py
index ffb27cd08d8..dee6dff08fb 100644
--- a/odoo/addons/test_testing_utilities/models.py
+++ b/odoo/addons/test_testing_utilities/models.py
@@ -211,3 +211,25 @@ class M2OOnchangeLine(models.Model):
     @api.onchange('dummy')
     def _onchange_flag(self):
         self.flag = True
+
+class O2MChangeCount(models.Model):
+    _name = 'test_testing_utilities.onchange_count'
+    _description = _name
+
+    count = fields.Integer()
+    line_ids = fields.One2many('test_testing_utilities.onchange_count_sub', 'parent')
+
+    @api.onchange('count')
+    def _onchange_count(self):
+        Sub = self.env['test_testing_utilities.onchange_count_sub']
+        recs = Sub
+        for i in range(self.count):
+            recs |= Sub.new({'name': str(i)})
+        self.line_ids = recs
+
+class O2MChangeSub(models.Model):
+    _name = 'test_testing_utilities.onchange_count_sub'
+    _description = _name
+
+    parent = fields.Many2one('test_testing_utilities.onchange_count')
+    name = fields.Char()
diff --git a/odoo/addons/test_testing_utilities/tests/test_form_impl.py b/odoo/addons/test_testing_utilities/tests/test_form_impl.py
index 344ae62f4d2..c13dbd73a60 100644
--- a/odoo/addons/test_testing_utilities/tests/test_form_impl.py
+++ b/odoo/addons/test_testing_utilities/tests/test_form_impl.py
@@ -432,6 +432,40 @@ class TestO2M(TransactionCase):
         with f.line_ids.edit(index=0) as new_line:
             self.assertTrue(new_line.flag)
 
+    def test_o2m_remove(self):
+        def commands():
+            return [c[0] for c in f._values['line_ids']]
+        f = Form(self.env['test_testing_utilities.onchange_count'])
+
+        self.assertEqual(f.count, 0)
+        self.assertEqual(len(f.line_ids), 0)
+
+        f.count = 5
+        self.assertEqual(f.count, 5)
+        self.assertEqual(len(f.line_ids), 5)
+
+        f.count = 2
+        self.assertEqual(f.count, 2)
+        self.assertEqual(len(f.line_ids), 2)
+
+        f.count = 4
+
+        r = f.save()
+
+        previous = r.line_ids
+        self.assertEqual(len(previous), 4)
+
+        with Form(r) as f:
+            f.count = 2
+            self.assertEqual(commands(), [0, 0, 2, 2, 2, 2], "Should contain 2 creations and 4 deletions")
+        self.assertEqual(len(r.line_ids), 2)
+
+        with Form(r) as f:
+            f.line_ids.remove(0)
+            self.assertEqual(commands(), [2, 1])
+            f.count = 1
+            self.assertEqual(commands(), [0, 2, 2], "should contain 1 '0' command and 2 deletions")
+        self.assertEqual(len(r.line_ids), 1)
 
 class TestEdition(TransactionCase):
     """ These use the context manager form as we don't need the record
diff --git a/odoo/tests/common.py b/odoo/tests/common.py
index 447c6ace2f0..dd808aaec30 100644
--- a/odoo/tests/common.py
+++ b/odoo/tests/common.py
@@ -1435,12 +1435,12 @@ class Form(object):
                 return []
 
             v = []
-            c = {t[1]: t[2] for t in current if t[0] == 1} if current else {}
+            c = {t[1]: t[2] for t in current if t[0] in (1, 2)} if current else {}
             # which view should this be???
             subfields = descr['views']['edition']['fields']
             for command in value:
-                # TODO: get existing sub-values so we can pass them along?
                 if command[0] in (0, 1):
+                    c.pop(command[1], None) # remove record from currents
                     v.append((command[0], command[1], {
                         k: self._cleanup_onchange(
                             subfields[k], v, None
@@ -1448,10 +1448,14 @@ class Form(object):
                         for k, v in command[2].items()
                         if k in subfields
                     }))
+                elif command[0] == 2:
+                    v.append((2, command[1], False))
                 elif command[0] == 4:
-                    v.append((1, command[1], c.get(command[1], {})))
+                    v.append((1, command[1], c.pop(command[1], {})))
                 elif command[0] == 5:
                     v = []
+            # explicitly mark all non-relinked (or modified) records as deleted
+            for id_ in c: v.append((2, id_, False))
             return v
         elif descr['type'] == 'many2many':
             # onchange result is a bunch of commands, normalize to single 6