I use vim in an xterm. Just recently I have noticed the appearance of thin vertical highlighted bars on some lines at extreme right side of xterm. They are not tied to the text, for they remain in the same relative positions when I scroll the text, and they are outside vim's text area for I cannot move the cursor onto one.
They are about half the width of the cursor, and about half again as high, so there is no gap between 2 consecutive such. They appear and disappear for no apparent reason, and seem to have no purpose or significance.
Could someone kindly explain what they are?