How could we go about to draw a straight line (vertical or horizontal) on top of any other window on our Linux desktop? I'm using i3 window manager with picom compositor at present, so I'm primarily interested in solutions that works for that.

I should get back to this question in the future and implement a nicer solution than pango-view

pango-view

Produces a vertical line (actually, more of a vertical box of limited width).
Very easy to make it, just issue the command in a terminal. pango-view was already installed.

$ pango-view --height=99999 --margin=1 --background=red -t ''

• https://forum.puppylinux.com/viewtopic.php?p=76743 relevant, very good and recent
• https://askubuntu.com/questions/328543/are-there-any-tools-for-drawing-on-the-desktop-drawing-over-apps
• gromit-mpx is a good over-app painter, but does not provide perfectly straight lines
• https://www.linuxfordevices.com/tutorials/linux/tools-to-draw-on-linux-screen