What do you call a very long line of the same character, which happens when the button is being continuously pressed? It may be accidentally or on purpose (for a visual effect). I've tried googling
long chain of characters or
stack key but neither is correct.
Lengthening a vowel sound for aural or visual effect.
The explanation given in this blog is the right idea but poorly expressed because it is the vowel sounds only that make the word longer when spoken: I Can Lengthen My Words and So Can Youuuuuuu
Visually, it would be: repeating a single character repeatedly for visual effect.
The two ideas are not in the same space.
I do this aaaaaalllll the time, usually with awesoooommmmeee! My friends now even say 'awesomie' verbally which is a bit wierd!
I'm borrowing 'concatenated' which is used in computing where strings of characters are glued together for various reasons into data strings.
Concatenate means 'to link things together in a chain or series'.
I'm saying that the repeated letters are joined together, like a chain.
loooooooooong chaiiiin of character ...
I second Andrew's statement: I guess there is no word for it. By the way, in German there is no word for it too.
Note: I am neither a native speaker nor an expert - I am just trying to help.