When you now run developer support, the toolbar is not initialized => the CSS is not loaded. But the special developer support div tags for the showing and hiding of the ADP files are still there, because this is something inherent to the ADP code.
This all leads to a page which is not viewable. If you change this in HEAD, then we should be fine, but for the time being it looks like (from experience) that the latest commit to HEAD puts the initialization of developer support and .LRN support into site-master. And until this is reflected in lrn-master you need to change it to site-compat instead of blank-compat. Hope this clarifies the situation.