NumberBox: Fix tabIndex attribute not set on init (T1275314) (DevExpress#29083)

Author: nikkithelegendarypokemonster

packages/devextreme/js/__internal/ui/number_box/m_number_box.base.ts

@@ -109,6 +109,7 @@ const NumberBoxBase = TextEditor.inherit({
     this.$element().addClass(WIDGET_CLASS);
 
     this.callBase();
+    this._toggleTabIndex();
   },
 
   _getDefaultAttributes() {

packages/devextreme/testing/tests/DevExpress.ui.widgets.editors/numberBoxParts/common.tests.js

@@ -67,6 +67,27 @@ QUnit.module('basics', {}, () => {
         assert.equal($element.find('.' + INPUT_CLASS).attr('inputmode'), 'decimal', 'inputmode is correct');
     });
 
+    QUnit.module('Tabindex', () => {
+        [false, true].forEach((initialFocusStateEnabled) => {
+            QUnit.test(`should be ${initialFocusStateEnabled ? 0 : -1} if focusStateEnabled = ${initialFocusStateEnabled} on init (T1275314)`, function(assert) {
+                const $element = $('#numberbox').dxNumberBox({ focusStateEnabled: initialFocusStateEnabled });
+                const $input = $element.find(`.${INPUT_CLASS}`);
+
+                assert.strictEqual($input.prop('tabindex'), initialFocusStateEnabled ? 0 : -1);
+            });
+
+            QUnit.test(`should be updated when focusStateEnabled changes from ${initialFocusStateEnabled} to ${!initialFocusStateEnabled}`, function(assert) {
+                const $element = $('#numberbox').dxNumberBox({ focusStateEnabled: initialFocusStateEnabled });
+                const instance = $element.dxNumberBox('instance');
+                const $input = $element.find(`.${INPUT_CLASS}`);
+
+                instance.option('focusStateEnabled', !initialFocusStateEnabled);
+
+                assert.strictEqual($input.prop('tabindex'), initialFocusStateEnabled ? -1 : 0, 'Updated tabindex is correct');
+            });
+        });
+    });
+
     QUnit.test('onContentReady fired after the widget is fully ready', function(assert) {
         assert.expect(2);