Fix posn-at-point in Flycheck buffers

* src/dispnew.c (buffer_posn_from_coords): Improve commentary.

* src/xdisp.c (move_it_in_display_line_to): Don't exit the loop
under truncate-lines if the glyph at TO_CHARPOS was not yet
produced.  This avoids bailing out too early when we are at
TO_CHARPOS, but didn't yet produce glyphs for that buffer
position, because the last call to PRODUCE_GLYPHS at this position
was for an object other than the buffer.  For further details, see
http://lists.gnu.org/archive/html/emacs-devel/2018-01/msg00537.html.

(cherry picked from commit c0154ac7c3423f68d8f3a2e85a756c9759219039)

2 files changed, 15 insertions, 1 deletions

diff --git a/src/dispnew.c b/src/dispnew.c
index ae6799bb85c..a81d6f64d1e 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -5208,6 +5208,11 @@ buffer_posn_from_coords (struct window *w, int *x, int *y, struct display_pos *p
 #ifdef HAVE_WINDOW_SYSTEM
   if (it.what == IT_IMAGE)
     {
+      /* Note that this ignores images that are fringe bitmaps,
+         because their image ID is zero, and so IMAGE_OPT_FROM_ID will
+         return NULL.  This is okay, since fringe bitmaps are not
+         displayed in the text area, and so are never the object we
+         are interested in.  */
       img = IMAGE_OPT_FROM_ID (it.f, it.image_id);
       if (img && !NILP (img->spec))
         *object = img->spec;
diff --git a/src/xdisp.c b/src/xdisp.c
index d6aabd06189..479a4c5a311 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -8790,7 +8790,16 @@ move_it_in_display_line_to (struct it *it,
 
       if (it->line_wrap == TRUNCATE)
         {
-          if (BUFFER_POS_REACHED_P ())
+          /* If it->pixel_width is zero, the last PRODUCE_GLYPHS call
+             produced something that doesn't consume any screen estate
+             in the text area, so we don't want to exit the loop at
+             TO_CHARPOS, before we produce the glyph for that buffer
+             position.  This happens, e.g., when there's an overlay at
+             TO_CHARPOS that draws a fringe bitmap.  */
+          if (BUFFER_POS_REACHED_P ()
+              && (it->pixel_width > 0
+                  || IT_CHARPOS (*it) > to_charpos
+                  || it->area != TEXT_AREA))
             {
               result = MOVE_POS_MATCH_OR_ZV;
               break;