I did quite a bit of searching and didn't find an identical problem. My issue is that when I open the template builder, the page becomes totally unresponsive (I'm working in Safari and using a child theme). No scrolling, clicking, anything. Everything else works just fine, pages are appearing as they should, etc. The whole Wordpress dashboard is AOK except for the Template Builder.
Thanks for any help you can provide!
(I saw another thread with a vaguely related issue that said it may be a plugin conflict, so deactivated plugin culprits with no luck.)