Each tool in the sidebar now has a star at the right of its row. It appears when you point at the row or reach it with the keyboard, and is always shown on touch screens. Select it and the tool joins the end of Your tools straight away; select a filled star to unpin. Pins are saved to your account for the current Personal or team selection, the same pins you manage with Customize on Dashboard. If a change cannot be saved, the sidebar goes back and says so.
The controls that hide the sidebar and keep it open now use the same sidebar icon, without a border.