@vicmn , that second paragraph is correct except that this is not just capital gain/ loss that affects the taxable portion/ amount of SSA - it is 1/2 of Social Security Benefit(s) plus all other income(s) compared to a threshold to determine the taxable portion. And you are right also that the capital gain tax rate is also based on gain compared active/ other income( s) for selecting 0%, 15% , etc. rate of taxation of the gain.
There is a worksheet ( go to forms mode ) which computes the taxable portion of the SSA benefits.
