Typically, clicking in the empty space at the bottom of a list view moves the keyboard focus to that list view. For example, clicking in the empty space at the bottom of the sidebar moves the keyboard focus to the sidebar, so that the up and down arrow keys can be used to move between the feeds.
Clicking in the empty space of the middle column (for example, if there are only a couple of visible articles, clicking below the bottom one) moves focus away from the sidebar, but it does not move focus into the middle column, so that the arrows keys cannot be used to cycle between articles.
Typically, clicking in the empty space at the bottom of a list view moves the keyboard focus to that list view. For example, clicking in the empty space at the bottom of the sidebar moves the keyboard focus to the sidebar, so that the up and down arrow keys can be used to move between the feeds.
Clicking in the empty space of the middle column (for example, if there are only a couple of visible articles, clicking below the bottom one) moves focus away from the sidebar, but it does not move focus into the middle column, so that the arrows keys cannot be used to cycle between articles.