From Tableau 2018.2, when you add a Phone or Tablet layout, Tableau automatically rearranges the Dashboard layout, making it ready to use. In the previous versions, you had to reorder it yourself. 

Consider the automatic Phone layout created by Tableau before 2018.2, as shown in the following screenshot:

The following is the same layout, automatically built by Tableau in 2018.2:

If you decide to change something inside of your custom layout, you can use the Rearrange layout option. It will automatically reorder the items and change the sizes. The option is available when you click on the three dots (...) next to a layout, highlighted as follows:

This feature doesn't allow you to create new things, but it'll save you a lot of time. The next feature is similar.