Fix 'vertical-motion' and 'posn-at-point' under 'visual-line-mode'

* src/xdisp.c (move_it_in_display_line_to): Don't assume we can
wrap on a whitespace character if it's followed by another
whitespace character.  When returning under WORD_WRAP for a screen
line that is continued, restore to wrap point when atpos/atx
position would be displayed on the next screen line due to
line-wrap.  (Bug#23570)

src/xdisp.c

@@ -8810,6 +8810,8 @@ move_it_in_display_line_to (struct it *it,
 			      ? WINDOW_LEFT_FRINGE_WIDTH (it->w)
 			      : WINDOW_RIGHT_FRINGE_WIDTH (it->w)))))
 		{
+		  bool moved_forward = false;
+
 		  if (/* IT->hpos == 0 means the very first glyph
 			 doesn't fit on the line, e.g. a wide image.  */
 		      it->hpos == 0
@@ -8828,16 +8830,37 @@ move_it_in_display_line_to (struct it *it,
 			     now that we know it fits in this row.  */
 			  if (BUFFER_POS_REACHED_P ())
 			    {
+			      bool can_wrap = true;
+
+			      /* If we are at a whitespace character
+				 that barely fits on this screen line,
+				 but the next character is also
+				 whitespace, we cannot wrap here.  */
+			      if (it->line_wrap == WORD_WRAP
+				  && wrap_it.sp >= 0
+				  && may_wrap
+				  && IT_OVERFLOW_NEWLINE_INTO_FRINGE (it))
+				{
+				  struct it tem_it;
+				  void *tem_data = NULL;
+
+				  SAVE_IT (tem_it, *it, tem_data);
+				  set_iterator_to_next (it, true);
+				  if (get_next_display_element (it)
+				      && IT_DISPLAYING_WHITESPACE (it))
+				    can_wrap = false;
+				  RESTORE_IT (it, &tem_it, tem_data);
+				}
 			      if (it->line_wrap != WORD_WRAP
 				  || wrap_it.sp < 0
-				  /* If we've just found whitespace to
-				     wrap, effectively ignore the
-				     previous wrap point -- it is no
-				     longer relevant, but we won't
-				     have an opportunity to update it,
-				     since we've reached the edge of
-				     this screen line.  */
-				  || (may_wrap
+				  /* If we've just found whitespace
+				     where we can wrap, effectively
+				     ignore the previous wrap point --
+				     it is no longer relevant, but we
+				     won't have an opportunity to
+				     update it, since we've reached
+				     the edge of this screen line.  */
+				  || (may_wrap && can_wrap
 				      && IT_OVERFLOW_NEWLINE_INTO_FRINGE (it)))
 				{
 				  it->hpos = hpos_before_this_char;
@@ -8880,6 +8903,7 @@ move_it_in_display_line_to (struct it *it,
 				  result = MOVE_POS_MATCH_OR_ZV;
 				  break;
 				}
+			      moved_forward = true;
 			      if (BUFFER_POS_REACHED_P ())
 				{
 				  if (ITERATOR_AT_END_OF_LINE_P (it))
@@ -8907,7 +8931,14 @@ move_it_in_display_line_to (struct it *it,
 		     longer relevant, but we won't have an opportunity
 		     to update it, since we are done with this screen
 		     line.  */
-		  if (may_wrap && IT_OVERFLOW_NEWLINE_INTO_FRINGE (it))
+		  if (may_wrap && IT_OVERFLOW_NEWLINE_INTO_FRINGE (it)
+		      /* If the character after the one which set the
+			 may_wrap flag is also whitespace, we can't
+			 wrap here, since the screen line cannot be
+			 wrapped in the middle of whitespace.
+			 Therefore, wrap_it _is_ relevant in that
+			 case.  */
+		      && !(moved_forward && IT_DISPLAYING_WHITESPACE (it)))
 		    {
 		      /* If we've found TO_X, go back there, as we now
 			 know the last word fits on this screen line.  */
@@ -9088,9 +9119,18 @@ move_it_in_display_line_to (struct it *it,
 
 #undef BUFFER_POS_REACHED_P
 
-  /* If we scanned beyond to_pos and didn't find a point to wrap at,
-     restore the saved iterator.  */
-  if (atpos_it.sp >= 0)
+  /* If we scanned beyond TO_POS, restore the saved iterator either to
+     the wrap point (if found), or to atpos/atx location.  We decide which
+     data to use to restore the saved iterator state by their X coordinates,
+     since buffer positions might increase non-monotonically with screen
+     coordinates due to bidi reordering.  */
+  if (result == MOVE_LINE_CONTINUED
+      && it->line_wrap == WORD_WRAP
+      && wrap_it.sp >= 0
+      && ((atpos_it.sp >= 0 && wrap_it.current_x < atpos_it.current_x)
+	  || (atx_it.sp >= 0 && wrap_it.current_x < atx_it.current_x)))
+    RESTORE_IT (it, &wrap_it, wrap_data);
+  else if (atpos_it.sp >= 0)
     RESTORE_IT (it, &atpos_it, atpos_data);
   else if (atx_it.sp >= 0)
     RESTORE_IT (it, &atx_it, atx_data);