Dashed line on top and below a window when window scrolling is required

Question

In Cinnamon deskop, how to remove this dashed line, that supposedly wants to indicate that the items in the window are not all visible and you can scroll up or down? For example in nemo when there is a big folder where u need to scroll down, in the bottom of the white area is a dashed line. Same in all windows.

A screenshot example of system settings:

Answer 1

The self-answer by the OP didn't work for me on Xubuntu 18.04. E.g. in xfce4-panel's Whisker Menu I still had the dashed lines.

Instead, the answer from AskUbuntu worked for me. But I didn't edit the system theme file (this would require root access), instead creating ~/.config/gtk-3.0/gtk.css to contain the following:

scrolledwindow undershoot.top,
scrolledwindow undershoot.right,
scrolledwindow undershoot.bottom,
scrolledwindow undershoot.left { background-image: none; }

Now neither Whisker Menu, nor Mousepad have any dashed lines.

Answer 2

Found an answer! From Arch wiki:

Remove overlay scroll indicators

The positions of the overlay scrollbars are indicated by thin dashed lines in the application window. These dashed lines will be present even when overlay scrolling is disabled using the environment variable discussed in the section above. To remove the indicator lines, create the following file:

~/.config/gtk-3.0/gtk.css

/* Remove dotted lines from GTK+ 3 applications */
.undershoot.top, .undershoot.right, .undershoot.bottom, .undershoot.left { background-image: none; }