来自 RMXP 的 F1

B
The window frame and scroll arrows. The four corners are drawn at a set size of 16 x 16 pixels, and the remaining frame tiles at a width of 16 pixels to match the window's size. The arrows are used to indicate that scrolling is possible in a given window.

当窗口内容大小大于窗口大小时，会使用箭头来表明相应方向可以滚动显示。