New
#1
They're just indicating that the program is open and minimized. I've no idea how to remove it but it's a feature I really like.
I hate the lines. They are on my taskbar also.
They were not there when I upgraded to Windows 10 from 8.1.
Now that I have done a clean install the lines are irritatingly visible.
Hoping for help on this like the first poster.
All the best.
The line is there to see if the program is open.
To get rid of it close the program.
They're especially useful if you have several apps pinned to the taskbar.