ChatUtil is capable of copying and pasting texts in your chat box.
Now downloads are available in curseforge: https://www.curseforge.com/minecraft/mc-mods/chatutil
Hold Shift and click/drag, you can select multiple lines.
Then press Ctrl+Shift+C to put these lines into your chipboard.
Use command /chat buffer <20-1024> to set the buffer size of your chat box.
Using regex can help you filter your messages.
Use /chat filter set <name> <regex> to define a expression with a name, then use /chat filter use <name> [buffer_filtered=true] to implement your expression. buffer_filtered option sets if game stores the filtered messages in buffer(default true).
In normal chats, use \{pos} to post your position quickly.
Feel free to use this mod anywhere, cuz it's released under license CC0-1.0 .
Fell free to report issues ;)