The edit summary length counter (which counts backwards from 255) counts characters, not bytes. However, in languages that heavily rely on unicode charaters, a character is usually more than a byte.
When saving an edit with a summary that is accepted by the counter but with more than 255 bytes, a part of the summary is cut off.
Ceterum censeo bug 4715 esse delendam.
Version: unspecified
Severity: trivial
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=4715
The db accepts 255 bytes. The php code sets the input maxlength to 250 (instead of 255) to account for some of that overhead, and then a javascript is used to adjust it to the real space.
This is a fixed duplicate, unless it is somehow not working right.
Is that a recent fix? On https://www.mediawiki.org/wiki/VisualEditor:Test it does not work as described by you (though it might be that the live version is out of date).
Oh, I was describing the comment for the normal editor. Yes, the comment counter for the visual editor seems wrong. Sorry.
Should use either $.fn.byteLimit (jquery.byteLimit) or $.byteLength (jquery.byteLenght).
byteLength is for calculating the byte count, byteLimit is an application of that but VE might need its own implementation.