A cage is a set of cursor positions. A rectangular portion of the VBT can be selected by pressing a mouse button down and dragging the cursor to the opposite corner of the area. When the button is pressed the current position, the first corner of the area, is saved. Each time the cursor is moved a new rectangle is drawn showing the area between the saved corner and the current position. When the button is released the second corner is saved.
Nothing is done with the region.
The last rectangle is not even erased, although normally it is no longer important. The rectangle is not redrawn, even if the application is revealed after being covered by another application. The repaint procedure would be responsible for maintaining the appearance of the selected region if, for example, it were to be redrawn or displayed in a different color. The following images Rubber 1 Rubber 2 depict two different views of the application, although there is not much to see. Two views are shown each with a different rectangle.