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https://slackalaxy.com/2021/01/07/gtk3-scrollbar-arrows/
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GTK3 scrollbar arrows

Posted: 2021-01-07 | Author: slackalaxy

I am not particularly happy with the lack of scrollbar arrows in
application windows. Unfortunatelly, this is the default behaviour of
GTK3 programs. I already dealt with GTK3 programs automatically hiding
the scrollbars, so I decided to get the arrows back, as well.

I searched a bit and found this solution in the Linux Mint forums. So,
following the instructions, I created ~/.config/gtk-3.0/gtk.css and
added the following contents:

/* Scrollbar width fixes */
scrollbar.vertical slider,
scrollbar.slider.vertical
{
	min-width: 1em;
}
scrollbar.horizontal slider,
scrollbar.slider.horizontal
{
    min-height: 1em;
}
 
/* Steppers */
* {
	-GtkScrollbar-has-backward-stepper: 1;
	-GtkScrollbar-has-forward-stepper: 1;
}
 
scrollbar button {
	min-width: 1em;
	min-height: 1em;
}
 
scrollbar.vertical button.down {
	-gtk-icon-source: -gtk-icontheme("pan-down-symbolic");
}
 
scrollbar.vertical button.up {
	-gtk-icon-source: -gtk-icontheme("pan-up-symbolic");
}
 
scrollbar.horizontal button.down {
	-gtk-icon-source: -gtk-icontheme("pan-end-symbolic");
}
 
scrollbar.horizontal button.up {
	-gtk-icon-source: -gtk-icontheme("pan-start-symbolic");
}
 
undershoot.top, undershoot.right, undershoot.bottom, undershoot.left {
	background-image: none;
}


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https://stackoverflow.com/questions/50414957/gtk3-0-scrollbar-on-treeview-scrolledwindow-css-properties-to-control-scrol
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I found the that CSS syntax was a little different for the slider. With
some experimentation I was able to override the width and height of the
slider component. The px units are required for these property values
and without them they are ignored.

slider {
	min-width: 28px;
	min-height: 80px;
}