Default: DISABLED. Use script menu to Enable per host. A movable semi-transparent toggle appears in the corner. ON: links open in background tabs, Shift=foreground, Alt=current
This is the opt-in version, that means in order to use the script you need to enable it in the script menu on a per-host basis.
Hover the bottom-right corner to reveal a single toggle button that forces links to open in background tabs. Red = OFF, green = ON. The setting is remembered per page (origin + path). When you visit a page again the settings are automatically reapplied to that page.
This version allows for moving the toggle on/off button by dragging it around. to reset the position, double click the button. If the place you dragged the button falls off the page when you resize it then the button is moved inward to maintain visibility.
This version of the script has a master toggle menu item that appears in the Tampermonkey extension's menu. The toggle functions on a hostname basis
Modifiers:
when open tabs in background is turned toggled to ON then the following modifiers apply:
Shift+click: open the link in a foreground tab
ALT+click: open the link in the current tab