UniversalLanguageSelector generates buttons with JavaScript like <a href="#"> which allows a middle click with the mouse witch opens a new tab on the same page.
Either fill href with a suitable value where possible or don't set the attribute href or don't use the element a.
Had fixed this in upstream - https://github.com/wikimedia/jquery.uls/issues/27
Updated it in Mediawiki ULS extension. see gerrit I1efc7022
More removed in gerrit I8eacaa45. Can you be bit more specific? Which buttons causing the issue?
Every <a href="#"> button cause this issue.
You removed two occurrence, but there are still buttons with this issue.
Examples:
lib/jquery.ime/jquery.ime.js
1396: .attr( 'href', '#' )
1434: .attr( 'href', '#' )
1526: '<a class="ime-name imeselector-toggle" href="#"></a>' +
resources/js/ext.uls.interface.js
263: href: '#',
UniversalLanguageSelector.hooks.php
190: 'href' => '#',
382: 'default' => "<a id='uls-preferences-link' href='#'></a>",
443: 'href' => '#',
UniversalLanguageSelector.hooks.php
190: 'href' => '#',
is currently problematic, because makeListItem() in core omits the <a> entirely without the attribute href.