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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; }