Specifies the size of scrollbar thumb in &quot; value units &quot; . A value of ( &quot; Scroll value maximum &quot; minus &quot; Scroll value minimum &quot; ) / 2 would result in a thumb which occupies half of the background area.
在“数值单位”中指定滚动条翻阅块的大小。（“最大滚动值”减去“最小滚动数值”）的值/2 可得到大小为背景区域一半的翻阅块。