When you press a key that opens a GUI, like 'e' for inventory,
and that GUI opens with a text box, like the chat box, or creative search box.
the key you pressed to open that gui will sometimes be typed into that textbox.
E.g. when pressing t for chat, sometimes t will appear in the textbox,
or pressing '/' for command input will sometimes double up to make "//" appear.
or, with creative with search tab selected, pressing e for inventory, sometimes the e will appear in the textbox
It's not a "ghost" character, the inserted character acts just as if you typed it.
It is not consistent.
Only tested the Creative inventory and the chat/command box.
happened with both a PS/2 Mechanical keyboard, and a USB Slim keyboard
This did not affect 1.12(.2)
This did affect some of the 1.13 snapshots
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Sorry about @unknown, it seems to be upset about something in your post but I've reopened it. I thought we fixed that :/