二级菜单设置为下拉弹出时，三级菜单设置成左侧不渲染展示

在左右布局下，如果二级菜单设置为下拉弹出模式， 三级菜单设置成左侧不能渲染展示，调整成顶部才可以

Start by reproducing the issue in the left-right layout with the second-level menu set to dropdown and the third level set to left, then compare it with the top setting that renders correctly. Trace the menu rendering entry point and identify why the left third-level menu is omitted; done means it renders in the stated configuration without regressing the top setting.

Written by the indexing model from the issue text.