Directions support in Main Window

Four directions of button layout are introduced to Main Window in skim 1.3.0. This "screenshot" was made from 4 separate screenshots in gimp. The top left one in this screenshot is in the left-to-right direction mode, which is the original/default one. The left bottom one is the opposite: right-to-left direction, while you can tell the other two are top-to-bottom and bottom-to-top direction respectively.