While it fixes the issue, I don't know if it's the cleanest way to do it
(repeatedly calling set-face-extend on the same faces every time an Org
buffer is opened/reverted seems a bit silly).


Also, while this is not part of the report because Erik's example
happens not to have any text between the block delimiters (#+begin_src
and #+end_src), I would like to re-emphasize that I'm pretty sure
org-block (and maybe org-quote and org-verse) should be defined with
:extend t unconditionally, to maintain the "rectangular" aspect the
blocks have in a browser once exported to HTML: