The problem was previously described in https://tracker.phpbb.com/browse/PHPBB3-16393, for phpBB 3.2/3.3, but create a ticket here - for Website project - should be a better idea.

A Front-End Dev Team Member replied : https://tracker.phpbb.com/browse/PHPBB3-16393?focusedCommentId=62917&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-62917

edit: I replied him, about IE11, after creating that ticket : https://tracker.phpbb.com/browse/PHPBB3-16393?focusedCommentId=62920&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-62920

The previous ticket is now closed, as the issue will be adressed here.