Distance from the center, if you the value is positive, the dot will be positioned below the tab's center
if the value is negative, then dot will be positioned above the tab's center, default set to 8
Returns the insets to apply when using this decoration on a box
that has contents, so that the contents do not overlap the edges
of the decoration. For example, if the decoration draws a frame
around its edge, the padding would return the distance by which
to inset the children so as to not overlap the frame.
Tests whether the given point, on a rectangle of a given size,
would be considered to hit the decoration or not. For example,
if the decoration only draws a circle, this function might
return true if the point was inside the circle and false
otherwise.