Answer
Tap the toast to open the message, dismiss it, or let it close after ten seconds. On wide layouts it appears over the chat pane. Notification delivery still depends on your browser and permission settings.
On desktop browsers, the tab title can blink while the tab is hidden, People chats are unread and the notification count is positive. Returning stops the blinking. Reading every qualifying chat stops it after read state syncs. The displayed number is a notification count, not an exact unread-message count. A background app or hidden tab does not consume unread activity.