patch 8.2.1058: multiline conceal causes display errors

Problem:    Multiline conceal causes display errors.
Solution:   Do not allow conceal cross over EOL. (closes #6326, closes #4854,
            closes #6302)

src/testdir/test_conceal.vim

@@ -254,4 +254,26 @@ func Test_conceal_cursor_pos()
   call delete('XTest_conceal_curpos')
 endfunc
 
+func Test_conceal_eol()
+  new!
+  setlocal concealcursor=n conceallevel=1
+  call setline(1, ["x", ""])
+  call matchaddpos('Conceal', [[2, 1, 1]], 2, -1, {'conceal': 1})
+  redraw!
+
+  call assert_notequal(screenchar(1, 1), screenchar(2, 2))
+  call assert_equal(screenattr(1, 1), screenattr(1, 2))
+  call assert_equal(screenattr(1, 2), screenattr(2, 2))
+  call assert_equal(screenattr(2, 1), screenattr(2, 2))
+
+  set list
+  redraw!
+
+  call assert_equal(screenattr(1, 1), screenattr(2, 2))
+  call assert_notequal(screenattr(1, 1), screenattr(1, 2))
+  call assert_notequal(screenattr(1, 2), screenattr(2, 1))
+
+  set nolist
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/testdir/test_diffmode.vim

@@ -1118,4 +1118,16 @@ func Test_diff_rnu()
   call delete('Xtest_diff_rnu')
 endfunc
 
+func Test_diff_multilineconceal()
+  new
+  diffthis
+
+  new
+  call matchadd('Conceal', 'a\nb', 9, -1, {'conceal': 'Y'})
+  set cole=2 cocu=n
+  call setline(1, ["a", "b"])
+  diffthis
+  redraw
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab