The offset was given as a hardcoded (physical) pixel value and did not
take scaling into account. This lead to it being shifted closer towards
the icon as the scale increased.
This now replaces the hardcoded value with a CSS property which
automatically includes the scale factor.
Further this allows simplifying some calculations that previously were
trying to counteract the hardcoded offset using margins by using the
intended offset directly.
With this the dot in the dash is also now placed at exactly the bottom
of the hover highlight, while previously there was an unintentional 1px
space.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7488
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3245>
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