[FIX] web: wait many2many tags' fetches for rendering

Before this commit, we don't assure fetch's deferreds were resolved to
return _readGroup. Thus, in some conditions, kanban was rendered before
records data was updated so many2many tags could be missing.
Now, we return _readGroup when fetch's deferreds were resolved.

Task #1910805

closes odoo/odoo#29508

addons/web/static/src/js/views/basic/basic_model.js

@@ -4289,9 +4289,12 @@ var BasicModel = AbstractModel.extend({
                     }
                     return list;
                 }).then(function () {
-                    self._fetchX2ManysSingleBatch(list);
-                    self._fetchReferencesSingleBatch(list);
-                    return list;
+                    var fetchingDefs = [];
+                    fetchingDefs.push(self._fetchX2ManysSingleBatch(list));
+                    fetchingDefs.push(self._fetchReferencesSingleBatch(list));
+                    return $.when.apply($, fetchingDefs).then(function () {
+                        return list;
+                    });
                 });
             });
     },

addons/web/static/tests/views/kanban_tests.js

@@ -2378,6 +2378,51 @@ QUnit.module('Views', {
         kanban.destroy();
     });
 
+    QUnit.test('wait x2manys batch fetches to re-render', function (assert) {
+        assert.expect(4);
+        var done = assert.async();
+
+        var def;
+        var kanban = createView({
+            View: KanbanView,
+            model: 'partner',
+            data: this.data,
+            arch: '<kanban class="o_kanban_test">' +
+                        '<field name="product_id"/>' +
+                        '<templates><t t-name="kanban-box">' +
+                            '<div>' +
+                                '<field name="category_ids" widget="many2many_tags"/>' +
+                            '</div>' +
+                        '</t></templates>' +
+                    '</kanban>',
+            groupBy: ['product_id'],
+            mockRPC: function (route, args) {
+                var result = this._super(route, args);
+                if (args.method === 'read') {
+                    return $.when(def).then(function() {
+                        return result;
+                    });
+                }
+                return result;
+            },
+        });
+
+        def = $.Deferred();
+        assert.containsN(kanban, '.o_tag', 2);
+        kanban.update({groupBy: ['state']});
+
+        def.resolve().then(function () {
+            assert.containsN(kanban, '.o_tag', 2,
+                'Should display 2 tags after update');
+            assert.strictEqual(kanban.$('.o_kanban_group:eq(1) .o_tag').text(),
+                'gold', 'First category should be \'gold\'');
+            assert.strictEqual(kanban.$('.o_kanban_group:eq(2) .o_tag').text(),
+                'silver', 'Second category should be \'silver\'');
+            kanban.destroy();
+            done();
+        });
+    });
+
     QUnit.test('can drag and drop a record from one column to the next', function (assert) {
         assert.expect(9);