Working on programming a user interface so a user can select a line in a graph. I have a rotated mouse area to match that of the line to be selected. This mouse area can detect when a mouse enters the area. This seems to be working well. But then I noticed it was lopsided. Entering the area from one direction was not showing entry as soon as from another direction. Then I realized that the black border of the mouse was not being used to trigger entry into the area. It is the white area of the mouse tip that triggers this. The black area is only to provide contrast so you can see the mouse.

So on a mouse, and in mouse point selection on Ubuntu, the black border of the mouse only servers to provide contrast to the controlling white area of the mouse.

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