Adjustment handle
（重定向自Sizing handle）
In graphical user interfaces, the control element adjustment handle is a small box that appears on the corners and edges of a selected element, such as another graphical control element like a window, that let you change its size or shape.
By dragging and dropping the control handle onto an edge, you can make the control wider or narrower, taller or shorter. Corner handles let you move two edges at once.