On mobile, the nav bar back button is not clickable when a "block message" is displayed

[jeromecambon]

After the fix of #121, the back (arrow) button is no longer clickable.

To reproduce, using the mobile view

• expand the taxonomy navbar on the left of the page
• the back and home button are not clickable

See the following video below 👇🏿 or #163 (review)

208075831-47fff43b-be7d-42f4-9fc1-4af5dc87fd48.mp4

[jeromecambon]

The back button does not work even without block message.

[jeromecambon]

The issue is caused by:

• Message block is part of the toolbar
• z-index of the navigation is > z-index of toolbar (so that the navigation is above the message block)
• Since the back button is part of the toolbar, it cannot be clicked...

The clean solution would be to have a dedicated section for message block (so that it is not part of the toolbar), but it implies a lot of changes in the doc structure.
Regarding the limited impact of this issue, I am not sure it worth the effort.