zero-width space inbetween these: x​x
and inbetween two spaces: [ ​ ]
and at the end of the line: ​

non-breaking space: x x [   ]  

simple unicode: xµx [ µ ] µ

wide: xｗx [ ｗ ] ｗ

simple emoji: x👩x [ 👩 ] 👩

complex emoji: x👩‍🔬x [ 👩‍🔬 ] 👩‍🔬

Ｈｅｌｌｏ, ｗｏｒｌｄ!

line feed: xx [  ] 

vertical tab: xx [  ] 

horizontal tab: x	x [ 	 ]
this should be the longest line:
1234567	12345678	123456781234567812345678

Control character: xx [  ]