`beginning-of-visual-line' interacts poorly with our use of invisible
overlays and often moves point into the previous message or part even
though it looks like it's in the next message or part (e.g., if you
press C-a on a header line and the previous message is invisible,
point will move into the previous message, even though it appears to
still be on the header line).  This advises `beginning-of-visual-line'
to address this behavior.
---

This is a completely different approach that should fix the HTML
problem, as well as other problems.  I also have an alternate approach
to this that sets 'field properties on the header line and part
buttons, which has the same effect, but seems to have more
side-effects (e.g., if point is at the end of a part button, C-a won't
go to the beginning).  This patch, OTOH, strikes directly at the
strange (maybe even buggy) behavior of beginning-of-visual-line.  The
downside is that it's advice, and advice always feels a little
sketchy.

 emacs/notmuch-show.el |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 5751d98..6f5c53a 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -1120,6 +1120,28 @@ function is used."
     (notmuch-show-goto-first-wanted-message)
     (current-buffer)))
 
+(defadvice beginning-of-visual-line (around constrain-invisible activate)
+  "Fix point motion around invisible overlays.
+
+In `notmuch-show-mode', we often hide overlays that end with a
+newline (e.g., messages, parts, etc).  This has the effect of
+collapsing the overlayed text into the beginning of the next
+line.  Unfortunately, this causes `visual-motion' and hence
+`beginning-of-visual-line' to move to the first character of the
+invisible overlay rather than the more obvious first character of
+the visible line.  Visually, these two points are
+indistinguishable, but this tends to make actions mysteriously
+apply to the previous message when it looks like they should
+apply to the next message.
+
+This advice fixes this behavior in `notmuch-show-mode' by
+restricting the motion of `beginning-of-visual-line'."
+  (if (eq major-mode 'notmuch-show-mode)
+      (save-restriction
+	(narrow-to-region (line-beginning-position) (point-max))
+	ad-do-it)
+    ad-do-it))
+
 (defun notmuch-show-build-buffer ()
   (let ((inhibit-read-only t))
 
-- 
1.7.10.4