Evolution "View message source" results in blank or almost-blank window

Leap 15.0 running Evolution 3.26.6 against IMAP. Pressing CTRL-U to view a message source should result in exactly that, the message source. However, nothing is displayed in the message source window that opens, or occasionally, simply “:/” appears.

This issue is discussed on a Red Hat Fedora forums here:- https://bugzilla.redhat.com/show_bug.cgi?id=1560312
The issue has been reported to GNOME upstream here:- https://bugzilla.gnome.org/show_bug.cgi?id=794694 but GNOME blames WebkitGTK+ 2.2
There is, however, a workaround posted here:- https://gitlab.gnome.org/GNOME/evolution/commit/c903384a2c
I am unable to find whether WebGTK+ project has accepted the issue, which is claim,ed to be a regression from 2.18

Is there any way in which the workaround can be incorporated into LEAP 15’s version of Evolution, or has anyone got any ideas for any other workaround?