My great colleague Magnus Unger found a way to workaround this problem.
/sitecore/shell/Applications/Content Manager/Content Editor.js in the function
scContentEditor.prototype.editRichText (row 2399 in our 6.6 rev 130214 version, but this could vary), the first line looks like this after applying the Sitecore.Support.416400 patch:
var w = $('overlayWindow');
This doesn’t work in the Sitecore Desktop because the windows gets unique ID’s. By reverting this back into the original code that comes with a clean install, the code looks like this:
var w = $('overlayWindow') || Element.select(parent.document, '#' + frameElement.id + 'overlayWindow');
After changing this line back to its original, the Show editor buttons works again.