Thanks damyon for implementing MDL-58808, however the patch (https://github.com/moodle/moodle/commit/ae60722b14af0dbd664f5ef8ac1e4aa8ddbb72c3) is incomplete because it only applies the styles to the Boost theme.  The main purpose was to facilitate use of FontAwesome icons in the Navigation (and Administration) block(s) for any theme when '$THEME->iconsystem = \core\output\icon_system::FONTAWESOME;' was set in the theme config.php file.

So there is a style issue with:

.block_navigation .block_tree p.hasicon {
text-indent: -21px;
padding-left: 21px;
}

which when removed solves the problem.  Clearly this cannot be removed for when FontAwesome is not used and another possible selector introduced.