Steps to reproduce the issue

1. Create onContentBeforeDisplay plugin event without a fallback variable for the $page parameter. Like this:

public function onContentBeforeDisplay($context, &$row, &$params, $page) {
}

You can also temporary remove the = 0 after the $page for a existing plugin. For example the vote plugin on this line: https://github.com/joomla/joomla-cms/blob/staging/plugins/content/vote/vote.php#L62
2. Set the error reporting to maximum
3. Go to the media manager and notice the following error:

Too few arguments to function PlgContentVote::onContentBeforeDisplay(), 3 passed in /usr/local/apache2/htdocs/libraries/joomla/event/event.php on line 70 and exactly 4 expected

Expected result

No errors or warning

Actual result

Too few arguments to function PlgContentVote::onContentBeforeDisplay(), 3 passed in /usr/local/apache2/htdocs/libraries/joomla/event/event.php on line 70 and exactly 4 expected

System information (as much as possible)

PHP 7.1.7
Latest Joomla! staging

Additional comments

The issue also appear in a older PHP version (PHP7.x). But it seems like that the latest php version raises a error with higher priority than previous versions.

The error is caused by multiple event dipatchers in the administrator/components/com_media/views/medialist/tmpl folder that call the onContentBeforeDisplay event without the the fourth argument, $page. A solution might be adding a 0 as fourth parameter. However I'm not sure if this is the right solution.