I want to prevent TinyMCE disabling gecko spellcheck. I’ve added gecko_spellcheck : true to config. It works fine, but when I type a new paragraph all highlights of previous paragraph are gone. Some illustrations:

When I make right click on textarea and disable/enable spellchecking, all of errors are highlighted. How can I make TinyMCE always to highlight errors?

This is a problem that can be resolved.
If you reset the tinymce iframe body everything that is spelled incorrectly gets underlined.
You may use this
ed.getBody.innerHTML = ed.getBody.innerHTML;to make it work.