Visit this webpage in Ephy Tech Preview:
The page content loads, then a brief instant later the web view becomes fully white.
We've seen this before on at least one other website, but I'm not sure where the original bug reports is.
I can reproduce it every time with Epiphany TP, but not once with MiniBrowser in Debug or Release mode. This is odd.
Interesting, you're right that it doesn't happen in MiniBrowser, even with the Tech Preview versions of everything. Test:
$ flatpak run --devel --command=/bin/bash org.gnome.Epiphany
MiniBrowser handles it fine even with some tabs created.
It's caused by Epiphany's adblocker.
It's caused by the Fanboy Annoyance List, https://bugzilla.gnome.org/show_bug.cgi?id=796245