Math expressions get cut off at end of line

[whoinoi]

Steps to reproduce

1. Write a long math expression, i.e. $$ very long math expression $$.

Expected behaviour

The math expression (as well as text) should be displayed in its entirety.

Actual behaviour

Long single math expressions get cut off at the end.
On Firefox for Desktop only the expressions which are themselves longer than a whole line get cut off while smaller expressions which each fit in a line are distributed over several lines.
On Safari for iOS everything just gets cut off, even text.
(see screenshot below)

Browser/system information

macOS 10.13.2, Firefox 57.0.4
iOS 11.2.2, Mobile Safari

Additional details