There was a bug 60714 which was partially fixed by me in revision

There was a bug 60714 which was partially fixed by me in revision
2959. However the real problem was that once there was an update (by
folder selection or msg retrieval) in header-view for a folder whose
msg was shown in msg-view, the model referenced by msg-view was not
the same with the model in header-view. Because of this the dimming
for prev/next buttons could had not be synchronous with header-view.

There is an observer now. The observed is the meader-view and the
observer is the msg-view-window. The target is to detect if the
model of header-view is replaced. Another essential part is the
callback for row-insertions on the model. This can be found in
modest-msg-view-window.c file.

pmo-trunk-r3010