niggle (plural niggles)

A minor complaint or problem.

https://en.wiktionary.org/wiki/niggle

I'm not sure I understand your question.

Eat has its own major mode which is used when you open a standalone buffer via the eat function.

When it's embedded in Eshell it mostly just does the right thing whenever you invoke a command that uses terminal control codes (e.g. htop) -- and many of those can be closed with q, yes.

I assume Eat is activated for any program listed in the eshell-visual-commands variable (but I'll admit I don't really understand how that works). The notable new minor modes present when I run htop in eshell are Eat--Eshell-Local and Eat--Eshell-Process-Running.