The location sidebar in Seasonality is fully customizable when it comes to width. Just drag the gray divider between the sidebar and the main content area to make the sidebar bigger or smaller. If you want to hide the sidebar completely, drag the divider to the left edge of the window. Likewise, if you find that the location list isn’t showing as you would expect, drag the gray sidebar in from the edge of the window.
Make sure the window is large enough to show the sidebar. If the window isn’t wide enough, the sidebar will be hidden.