If no reason specified, open a book to allow the player to type a more detailed report

Another solution for the character limit would be allowing to use multiple lines. All chat messages of a player will be catched until he writes a word or command to exit this mode.

For example like this:
"you are now in report mode. Please write all needed details to the chat. You can use multiple lines. When you're done use the command /report done to exit report mode."

That would work much better in terms of cross-compatibility. Opening a book server side requires using packets/Protocol Lib. My only concern is if someone makes a mistake and wishes to edit a line.

If a report would allow the character limit of Minecraft only it would not be possible to edit the report afterwards as well.

True, but I meant to say that I think that people will not care about typos.

Perhaps, but I can imagine scenarios where people would, such as mentioning the wrong player names etc as that would affect the report itself. In those cases, it would be beneficial to offer a way to change a line whilst in 'report mode'.

I see....hm what about JSON'ing the chat?
Imagine this: You enter the first line of your report in report mode.
When sent to server with return the line you entered will be shown in chat and in theory you are now able to write the next line.

BUT. The text of the first line (or whatever line you just entered or show all lines) is not normal text. When moving the mouse over the text a box shows up with something like click to remove this line. And when you do, well, the line will be removed from the report ^^.

I know this way you cannot directly edit the line but I see no way to realize editable text using the chatbox. Though you can still do corrections in some way.

What if there were two options at the end of each line? One an edit, the other delete. Clicking edit puts the text in the command line, prefixed with #lineNumberHere textHere.

I did not know that it is possible to activate and feed a players chat line server side. That would be awesome.

It's not server side, it'd be done using JSON chat, which can prefill the required message.

Interesting to know anyways :P.

There would be even more possible applications for JSON chat.
For example you warn/kick/mute/ban a player for whatever reason and you have a typo within the reason. As all other players with appropriate permission you can see the message but for you as staff mate the message is JSONed so you can click on the reason and correct your typo.

Not the report as a whole, but on a line by line basis it would be possible.

In 1.11 they just increased the character limit from 100 to 256 characters