code: private void addScrollGapPath(int x, int y, int w, int h, boolean isAtLeft) {
          final double hHalf    = h / 2.0;
          final double wFull    = isAtLeft ? w : 0;
          final double wHalfOff = isAtLeft ? w - hHalf : hHalf;
  
          path.quadTo(x + wHalfOff, y + h, x + wHalfOff, y + hHalf);
          path.quadTo(x + wHalfOff, y, x + wFull, y);
      }
docs: Adds a hemispherical section to the current path. This is used to create
  the gap in a scroll bar button or cap into which the scroll bar thumb
  will fit.
  
  @param x        the X coordinate of the upper-left corner of the button
  or cap
  @param y        the Y coordinate of the upper-left corner of the button
  or cap
  @param w        the width of the button or cap
  @param h        the height of the button or cap
  @param isAtLeft {@code true} if the gap is at the left end of the button,
  {@code false} if it is at the right.