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)
diff --git a/src/testdir/test_matchadd_conceal.vim b/src/testdir/test_matchadd_conceal.vim
index 3dc6bb3..e6a5165 100644
--- a/src/testdir/test_matchadd_conceal.vim
+++ b/src/testdir/test_matchadd_conceal.vim
@@ -339,6 +339,27 @@
   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