mobile: improve behavior of sidebar menu

[nuc1eon]

I have noticed that when I open the sidebar menu

I can close it by tapping into the main field again.

However when I "drag" the sidebar to the left, the sidebar does not close correctly. Leaving only the transparent white overlay:

This can be tested on mobile and the browser inspector.

This does not happen on https://learn.getgrav.org/.

[McShelby]

This and #209 are serious sh... shnakeholes... grrr. Not fun! Learn is different but not necessarily better.

This will take some time.