If you open more articles, each article opens in own content tab. On the following screenshot, two articles are loaded, and the "bruno article 1" is in the foreground:

On the following screenshot, the "search engines" article is in the foreground. To switch between articles, just click the tab.

On the following screenshot, the articles are tiled vertically. To tile articles, right-click any of content tabs, and then select Tile vertically.

On the following screenshot, the articles are tiled horizontally. To switch from vertical to horizontal tiling, right-click any of content tabs, and then click Toggle Tile Orientation, or click Tile Horizontally. To return to overlapped tabs, click Untile.