That's due to the button you've added as it's overflowing to a new line with some margins.
I have the same issue - couldn't find a simple way to fix it.
Seems like this is going to be a reoccurring problem for the library, since material ui's snackbar by default has that button, while notistack removes it and requires the user to reinsert it.
That's due to the button you've added as it's overflowing to a new line with some margins.
It is overflowing but the styling should take care of to center the '.MuiSnackbarContent-action' regardless of the length of the text.
This is expected. Button will be on a new line as you can see on mat ui docs:
https://material-ui.com/components/snackbars/#message-length
But I agree in looks weird IF you're using an IconButton instead of Button.
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That's due to the button you've added as it's overflowing to a new line with some margins.