Wikimedia Bugzilla is closed!

When 'article' tab or 'talk' tab is selected and you select another, the tab
corresponding to the action becomes 'selected' (class="selected") but the
previous tab is still selected.

Think it could be near there:
SkinTemplate.php::SkinTemplate->buildContentActionUrls()

wfRunHooks( 'SkinTemplatePreventOtherActiveTabs', array( &$this ,
&$prevent_active_tabs ) )	;
if( $this->iscontent ) {
	$subjpage = $this->mTitle->getSubjectPage();
	$talkpage = $this->mTitle->getTalkPage();
	$nskey = $this->mTitle->getNamespaceKey();
	$content_actions[$nskey] = $this->tabAction(
		$subjpage,
		$nskey,
		!$this->mTitle->isTalkPage() && !$prevent_active_tabs,
		'', true);
	$content_actions['talk'] = $this->tabAction(
		$talkpage,
		'talk',
		$this->mTitle->isTalkPage() && !$prevent_active_tabs,
		'',
		true);