I have to apologize for the false alarm, the bug it's still there. I will pay more attention before posting, sorry.
Whatever caused the bug, it seems it's not reproducible anymore with WinSCP 5.0.5.
Tracker Bug 763 is still open so I suppose it was some sort of side effect.
Under one can't edit anymore I understand one can't ad any more content, however he can delete and replace content, but simply try to hit enter or insert a new char will not work.
While file is opened correctly, shown completely one can't edit anymore files bigger than 65535.
Tested with WinSCP v.5.04 build 1772 on windows xp 64