patch 8.2.4918: conceal character from matchadd() displayed too many times

Problem:    Conceal character from matchadd() displayed too many times.
Solution:   Check the syntax flag. (closes #10381, closes #7268)

src/drawline.c

@@ -2477,14 +2477,16 @@ win_line(
 
 #ifdef FEAT_CONCEAL
 	    if (   wp->w_p_cole > 0
-		&& (wp != curwin || lnum != wp->w_cursor.lnum ||
-						       conceal_cursor_line(wp))
+		&& (wp != curwin || lnum != wp->w_cursor.lnum
+						    || conceal_cursor_line(wp))
 		&& ((syntax_flags & HL_CONCEAL) != 0 || has_match_conc > 0)
 		&& !(lnum_in_visual_area
 				    && vim_strchr(wp->w_p_cocu, 'v') == NULL))
 	    {
 		char_attr = conceal_attr;
-		if ((prev_syntax_id != syntax_seqnr || has_match_conc > 1)
+		if (((prev_syntax_id != syntax_seqnr
+					   && (syntax_flags & HL_CONCEAL) != 0)
+			    || has_match_conc > 1)
 			&& (syn_get_sub_char() != NUL
 				|| (has_match_conc && match_conc)
 				|| wp->w_p_cole == 1)

src/testdir/test_matchadd_conceal.vim

@@ -339,6 +339,27 @@ func Test_matchadd_and_syn_conceal()
   call assert_equal(screenattr(1, 11) , screenattr(1, 32))
 endfunc
 
+func Test_interaction_matchadd_syntax()
+  CheckRunVimInTerminal
+
+  new
+  " Test for issue #7268 fix.
+  " When redrawing the second column, win_line() was comparing the sequence
+  " number of the syntax-concealed region with a bogus zero value that was
+  " returned for the matchadd-concealed region. Before 8.0.0672 the sequence
+  " number was never reset, thus masking the problem.
+  call setline(1, 'aaa|bbb|ccc')
+  call matchadd('Conceal', '^..', 10, -1, #{conceal: 'X'})
+  syn match foobar '^.'
+  setl concealcursor=n conceallevel=1
+  redraw!
+
+  call assert_equal('Xa|bbb|ccc', Screenline(1))
+  call assert_notequal(screenattr(1, 1), screenattr(1, 2))
+
+  bwipe!
+endfunc
+
 func Test_cursor_column_in_concealed_line_after_window_scroll()
   CheckRunVimInTerminal
 

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4918,
 /**/
     4917,
 /**/