[FIX] web: correctly update input field after onchange

In a form view, have an x2many list with two fields A and B (e.g.
A is a many2one, and B is a char). There is an onchange on the
x2many that is triggered when the user sets A, and that sets B.

In this scenario, let's assume that the user first writes something
in B, but directly deletes it. Then, he sets A. Before this commit,
in this situation, B wasn't updated with the value returned by the
onchange.

Indeed, B's field widget was still flagged as 'isDirty', because
the user interacted with it, but it didn't commit its value, as it
hasn't actually changed. Basically, it remained flagged as 'isDirty'
forever, and thus couldn't be updated with the value returned by
the onchange.

This commit ensures that the 'isDirty' status is correctly updated
when the change is actually undone.

Task 1958793

closes odoo/odoo#51085

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>
Co-authored-by: pka-odoo <pka@odoo.com>
Co-authored-by: Mohammed Shekha <msh@odoo.com>

addons/web/static/tests/fields/basic_fields_tests.js

@@ -1743,6 +1743,54 @@ QUnit.module('basic_fields', {
         form.destroy();
     });
 
+    QUnit.test('input field: set and remove value, then wait for onchange', async function (assert) {
+        assert.expect(2);
+
+        this.data.partner.onchanges = {
+            product_id(obj) {
+                obj.foo = obj.product_id ? "onchange value" : false;
+            },
+        };
+
+        let def;
+        const form = await createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: `
+                <form>
+                    <field name="p">
+                        <tree editable="bottom">
+                            <field name="product_id"/>
+                            <field name="foo"/>
+                        </tree>
+                    </field>
+                </form>`,
+            async mockRPC(route, args) {
+                const result = this._super(...arguments);
+                if (args.method === "onchange") {
+                    await Promise.resolve(def);
+                }
+                return result;
+            },
+            fieldDebounce: 1000, // needed to accurately mock what really happens
+        });
+
+        await testUtils.dom.click(form.$('.o_field_x2many_list_row_add a'));
+        assert.strictEqual(form.$('input[name="foo"]').val(), "");
+
+        await testUtils.fields.editInput(form.$('input[name="foo"]'), "test"); // set value for foo
+        await testUtils.fields.editInput(form.$('input[name="foo"]'), ""); // remove value for foo
+
+        // trigger the onchange by setting a product
+        await testUtils.fields.many2one.clickOpenDropdown('product_id');
+        await testUtils.fields.many2one.clickHighlightedItem('product_id');
+        assert.strictEqual(form.$('input[name="foo"]').val(), 'onchange value',
+            'input should contain correct value after onchange');
+
+        form.destroy();
+    });
+
     QUnit.module('UrlWidget');
 
     QUnit.test('url widget in form view', async function (assert) {