Apparently the ‘enter’ character creates a square in Textboxes if the ‘MultiLine’ boolean is set to true and the Font is set to anything that is not Code, Fantasy or any of the SourceSans fonts. Example:
Depending on the font the square icon also changes (obviously). Here’s a simple repro:
go into studio
Insert a ScreenGui with a TextBox
Set MultiLine to true
Set the Font to anything except for Code, Fantasy or SourceSans (any of them).
Start a test server
Start typing in the TextBox and press enter
As you can see, the enter character shows up even though it shouldn’t
Edit: tagging @spotco since I think he made the new fonts happen.
As @Vorlias said, it’s the \r character. Roblox works if you do JUST \n (Most newlines are represented by \r\n) - I’ve been able to make a workaround by changing the encoding of text that I copy/paste in Notepad++. I can’t recall which encoding I use though. Think it’s ASCII.