detach: bottom tangential grouping

Problem

• too thin bottom seg: when trying to group them, the threshold for overlapping, which relates to the thickness of the bottom seg, would be so low
  
• so that they never appear overlapped. because they are straight lines slightly misaligned radially.
  

Solution

• radial + tangential threshold
  
• add a counter-proportional factor by their tangential gap
• tangential gap++, thresh++
• absolute overlap has top priority