The clickEvent property of JSON text is supposed to allow you to trigger things when the displayed text is clicked. But when used with text for the /title command, it doesn't, though it displays fine otherwise and throws no errors.
To Reproduce:
1. Put this in a command block:
/title @a subtitle {"text":"Click here to continue.","underlined":true,"color":"blue","clickEvent":{"action":"run_command","value":"/say Oh, hey, I continued!"}}/title @a subtitle {"text":"Click here to continue.","underlined":true,"color":"blue","clickEvent":{"action":"run_command","value":"/say Oh, hey, I continued!"}}2. Trigger the command block.
3. While the subtitle is displayed, open the chat to unlock your mouse, and click the title text. Nothing happens, whereas the clickEvent clearly should trigger a message in the chat.
This is very likely intended or a feature request. The title would vanish at some point anyway and the user could not access it after that anymore.
There are other situations in which click and hover events do not work, for example the header and footer of the player list.