First reported by miya at ja.wp:
write something in Japanese; press Enter; the whole line disappears. This also happens when adding a picture - in the Caption field.
This does not seem to be browser or OS-specific.
miya says that adding a "single-byte character" or a number at the beginning of the line, the line doesn't disappear - I don't seem able to confirm this (my tests are in Safari/X Yosemite).
I'm able to reproduce the general issue at mediawiki.org (using the laptop input system for Katakana; hope I'm doing this right).
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=72508