On examples of documentation, on Custom Snackbar, if you open two snackbars and expend one, it will overlap the other

[conmeconte]

Expected Behavior

When one of the snackbar is expended, the other snackbar should reposition to not overlap with the expended snackbar

Current Behavior

When there are two expendable snackbars and one is expended, it overlaps the other

Steps to Reproduce

Go to https://iamhosseindhv.com/notistack/demos#custom-snackbar. On custom snackbar demo, click the button to open two snackbars(you might have to click twice within short time interval to have two snackbars on the screen).
Expend the bottom snack bar.
You will see that the expended snackbar is overlapped with the other snackbar.

Context

I have a need to display two expendable snackbars on the same screen, and because of this issue, the title or content of the individual snackbar is blocked.

Your Environment

Tech	Version
Notistack	v0.8.6
React	16.8.5
Browser	Chrome Version 74.0.3729.169 (Official Build) (64-bit)
etc.

[iamhosseindhv]

#167