i've seen the error a few times:
msgheaderElem.children[(msgheaderElem.children.length - 1)] is undefined
i've seen it happen after sending a reply (with the "answered" flag added).
the updateKeywords callback would render the message again, but the code for
rendering the "additional headers" table rows again was making invalid
assumptions.
the approach is now changed. the backend now just immediately sends the
additional headers to the frontend. before, the frontend would first render the
base message, then render again once the headers came in for the parsed
message. this also prevents a reflow for the (quite common) case that one of
the additional headers are present in the message.
1e15a10b66