numix-gtk-theme/gtk-3.0/scss/widgets/_overshoot.scss

@mixin overshoot($position, $type: normal, $color: $selected_bg_color) {
    $_small_gradient_length: 5%;
    $_big_gradient_length: 100%;

    $_position: center top;
    $_small_gradient_size: 100% $_small_gradient_length;
    $_big_gradient_size: 100% $_big_gradient_length;

    @if $position == bottom {
        $_position: center bottom;
        $_linear_gradient_direction: to top;
    } @else if $position == right {
        $_position: right center;
        $_small_gradient_size: $_small_gradient_length 100%;
        $_big_gradient_size: $_big_gradient_length 100%;
    } @else if $position == left {
        $_position: left center;
        $_small_gradient_size: $_small_gradient_length 100%;
        $_big_gradient_size: $_big_gradient_length 100%;
    }

    $_small_gradient_color: $color;
    $_big_gradient_color: $color;

    $_small_gradient: -gtk-gradient(radial,
                                    $_position, 0,
                                    $_position, .5,
                                    to(fade-out($_small_gradient_color, .75)),
                                    to(fade-out($_small_gradient_color, .85)));

    $_big_gradient: -gtk-gradient(radial,
                                  $_position, 0,
                                  $_position, .6,
                                  from(fade-out($_big_gradient_color, .8)),
                                  to(fade-out($_big_gradient_color, 1)));

    @if $type == normal {
        background-image: $_small_gradient, $_big_gradient;
        background-size: $_small_gradient_size, $_big_gradient_size;
    } @else if $type == backdrop {
        background-image: $_small_gradient;
        background-size: $_small_gradient_size;
    }

    background-repeat: no-repeat;
    background-position: $_position;

    background-color: transparent; // reset some properties to be sure to not inherit them somehow
    border: 0;
    box-shadow: none;
}

@mixin undershoot($position) {
    $_undershoot_color_dark: fade-out($black, .8);
    $_undershoot_color_light: fade-out($white, .8);

    $_gradient_dir: left;
    $_dash_bg_size: 10px 1px;
    $_gradient_repeat: repeat-x;
    $_bg_pos: center $position;

    background-color: transparent; // shouldn't be needed, but better to be sure;

    @if ($position == left) or ($position == right) {
        $_gradient_dir: top;
        $_dash_bg_size: 1px 10px;
        $_gradient_repeat: repeat-y;
        $_bg_pos: $position center;
    }

    background-image: linear-gradient(to $_gradient_dir, // this is the dashed line
                                         $_undershoot_color_light 50%,
                                         $_undershoot_color_dark 50%);

    padding-#{$position}: 1px;
    background-size: $_dash_bg_size;
    background-repeat: $_gradient_repeat;
    background-origin: content-box;
    background-position: $_bg_pos;
}

// This is used by GtkScrolledWindow, when content is touch-dragged past boundaries.
// This draws a box on top of the content, the size changes programmatically.
.overshoot {
    &.top {
        @include overshoot(top);

        &:backdrop { @include overshoot(top, backdrop); }
    }

    &.bottom {
        @include overshoot(bottom);

        &:backdrop { @include overshoot(bottom, backdrop); }
    }

    &.left {
        @include overshoot(left);

        &:backdrop { @include overshoot(left, backdrop); }
    }

    &.right {
        @include overshoot(right);

        &:backdrop { @include overshoot(right, backdrop); }
    }
}

// Overflow indication, works similarly to the overshoot, the size if fixed tho.
.undershoot {
    &.top { @include undershoot(top); }

    &.bottom { @include undershoot(bottom); }

    &.left { @include undershoot(left); }

    &.right { @include undershoot(right); }
}