LOCATION
========
viewtopic.php, directly after the post loop ends (after
unset($rowset, $user_cache), at the "Update topic view and if
necessary attachment view counters" block. The existing condition is
moved into a variable so listeners can change it; the counting itself
is unchanged:

    unset($rowset, $user_cache);

    // Update topic view and if necessary attachment view counters ... but only for humans and if this is the first 'page view'
    $update_view_counters = isset($user->data['session_page']) && !$user->data['is_bot'] && (strpos($user->data['session_page'], '&t=' . $topic_id) === false || isset($user->data['session_created']));

    /**
    * Modify whether the topic view counter and attachment download
    * counters are updated for this page view
    *
    * @event core.viewtopic_modify_view_counter_update
    * @var bool        update_view_counters    Whether to increase the topic view count and the attachment download counts
    * @var array    update_count            IDs of the attachments whose download count will be increased
    * @var int        forum_id                Forum ID
    * @var int        topic_id                Topic ID
    * @var array    topic_data                Array with topic data
    * @since to be filled in by the phpBB dev team at merge time
    */
    $vars = array(
        'update_view_counters',
        'update_count',
        'forum_id',
        'topic_id',
        'topic_data',
    );
    extract($phpbb_dispatcher->trigger_event('core.viewtopic_modify_view_counter_update', compact($vars)));

    if ($update_view_counters)
    {
        $sql = 'UPDATE ' . TOPICS_TABLE . '
            SET topic_views = topic_views + 1, topic_last_view_time = ' . time() . "
            WHERE topic_id = $topic_id";
        $db->sql_query($sql);

        // Update the attachment download counts
        if (count($update_count))
        {
            ...

The code at this spot is the same on 3.3.x and master, so the change
should merge forward without conflict.

PURPOSE
=======
Let extensions decide whether a page view adds to a topic's view count
(and to the download counts of attachments shown inline on that page).

USE CASE
========
Board owners have asked for years for some views not to count:

• an admin setting up forums, checking permissions or editing many
    imported posts adds hundreds of views to topics nobody else has read;
• a topic author can push up the view count of their own topic just by
    reloading it.

The only answer today is to edit the condition in viewtopic.php by hand
and redo the edit after every phpBB update. Recent example (phpBB
3.3.19): https://www.phpbb.com/community/viewtopic.php?p=16140973#p16140973

phpbbmodders/viewcountexempt will use this event to skip the count for
users or groups given a "views do not count" permission, for topic
authors on their own topics, and for admins, each set in the ACP.

No existing event can do this. core.viewtopic_post_row_after and the
other viewtopic events fire before the counter update but cannot change
its condition. The only workarounds are changing $user->data
['session_page'] so the check thinks the topic was already viewed (which
other code may read later), or taking the view back off with a second
UPDATE afterwards (an extra query on every topic view, and racy).

VARIABLES
=========

• update_view_counters (bool, read/write) - whether to increase the
    topic view count and the attachment download counts
• update_count (array, read/write) - IDs of the attachments whose
    download count will be increased; a listener can change it to count
    topic views but not downloads
• forum_id (int, read-only) - forum ID
• topic_id (int, read-only) - topic ID
• topic_data (array, read-only) - topic data, including topic_poster