Today I noticed that it's completely impossible to write any Flow posts through the mobile interface.
To reproduce, simply try typing a reply to any topic at https://m.mediawiki.org/wiki/Talk:CX (notice the m in the beginning). Start a new topic is broken, too: you can type the subject, but not the opening post. Nothing can be typed at all.
I tried it with Samsung Galaxy S5 with Firefox and Chrome, and with desktop Firefox on the mobile site (m).
On the non-mobile site it works correctly.