The iframe in mass_mailing is self-resizing in order to avoid having two
vertical scrollbar side by side. This failed since the conversion to OWL
due to the vertical offset added when resizing having become
insufficient. This adds a class to the <html> element of the iframe to
identify when it's in the context of mass_mailing so we can remove the
overflow visible style that is not needed in this context. This allows
us to remove that vertical offset altogether since the scrollbar
disappeared.
X-original-commit: 4f538911d68afdeca0253f15c9e8ffdbed4e995c
Part-of: odoo/odoo#102734
Antoine Guenet
8a74c1f793