After upgrading to 6.6 from 6.5.4 the Canvas dashboard is stuck on "Canvas is loading" if I'm under a "panel" (spaces). It works fine under the default "panel" (spaces).
When you say "panels", are you talking about "spaces"? Like in this interface:
If so, yes, this is a regression and a (now) known issue. You can track the bug here: https://github.com/elastic/kibana/issues/27262
I added a note in that issue but thought I'd updated here as well. Apparently the fix went in too late, so 6.6.0 is broken, but whenever 6.6.1 comes out, it should work again.