Hi again,
Everything is suitably patched and updated, yes.
Opera : 17.0.1241.53
WP 3.7.1, HTML editor 1.4.4
Windows 7 Pro, fully patched
Media settings in WP are fine too.
Hmm, not tried Safari, but IE, Chrome and Firefox all exhibit the same issue, even with caches emptied and all plug-ins except yours disabled. Also not caused by a framework or a specific site either. Just setup a test domain with a bare install and default WordPress theme. Same.
~ Paul