TinyMCE plugin (3.x & 4.x): switch Visual->Text and back, contents marked dirty

Working on a tinymce plugin to make a pretty rendering for a shortcode used by my WP plugin. Code is based on the wpeditimage tinymce plugin from WP core. Have equivalent code for tinymce 3.x and 4.x, and both show the same bug.

The bug: load an editor page. The “Visual” tab is selected and tinymce is shown, and my plugin has worked as expected so far. Select “Text” to get the plain textarea, my plugin still works because the original shortcode is shown. Select “Visual” again, and again all seems to be good. With no edits, navigating away from the page (e.g. selecting a left side menu item) gets the ‘Are You Sure?’ prompt, indicating that the content is considered changed!

The wpeditimage plugin does not show this bug, and I’ve spent ridiculous time studying that for something I missed — plus the tinymce code, of course. No joy.

Any clues?

I have a nearly minimal test plugin that shows the bug; I’ll put it up for DL if anyone wants to look.