How to get rid of minecraft (mineplex?) shortcut to words in text? Help?

I was playing on mineplex when some one stole my iron on a game, so in chat I held the shift button down, and pressed 'v' it automatically put something in the chat… How do I stop it? I reset the controls, but wht do I do? Because before all of this happened, It highlights my text then deletes it? I have vanilla minecraft? + no plugins… The first pic show when it high lights ITS SELF I don't click anything else when it happens. I typed something out of minecraft and it stopped hightlighting it, but what I typed on the internet comes up as a short cut in minecraft (when running it in the background as I type it?) I have reset the controls to no avail… Help! (I am on a macbook pro)

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