Hi

This is looking good: I have two comments the second of which is significant.

The first is do you want to sort (alphabetically) the headerline tags?
As it stands they are in the order they appear in the thread which is
probably not what is wanted.

The second is that there is a notmuch-show-tag-all functions to tag all
messages in the thread. Your patch is quadratic for the update (as it
calculates the list of thread tags once for each message). The attached
patch would avoid this and doesn't look too bad. (Note I retained
no-headerline-update as an optional argument in case there are out of
tree callers, eg users' .emacs files)

[Note this is not purely of academic interest: on my test large thread
(178 messages) updating the display after tagging all messages took some
seconds without my patch and almost no time with it.]

Best wishes 

Mark