Editing a book counts as idle

I think this is intended, as writing in a book in singleplayer effectively pauses the game, as if you were in the escape menu. However, this combined with the Realms timeout makes this an issue.

Technicly this is a suggestion

No, it's a bug, as the player is doing something and shouldn't be counted as idle.

Client doesn't provide any feedback for editing a book and there isn't really a reason to, unless you reworked a new packet specifically for "I'm editing a book don't kick me"

Is this still an issue in version 1.13.2 or the latest snapshot?

Recommended fix would be to have a warning message telling the player that they are going to get kicked if they are on realms. Maybe some GUI that you can  click on to prove you are at your computer for some sort of keep alive.