当用户在Action Bar中点击时，applet拦截此响应并指出用户点击了哪个按钮，给出它的边界。
When a user clicks inside of the Action Bar, the applet intercepts that click and figures out which button the user's mouse is over, given its boundaries.
图1：蓝色线框是可点击范围的边界。
Figure 1. The blue line represents the boundary of the clickable region.
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