Hi @rotendahl. I’m still fairly new to Obsidian so I’m not too clear yet on best practices, but perhaps something like this in a CSS snippet (Settings → Appearance) could work. It will only show in the current tab and only in editing / live preview mode (there’s likely no need to show the line in a pane where you’re not editing), though you could show it in all panes by removing .mod-active from the second CSS declaration.
You can make the line darker by increasing the percentage in background-color in the second declaration. You can make the line lighter in dark mode by increasing the percentage in background-color in the last declaration. Finally, you can change the breakpoint by adjusting left: 80ex;.