vim-patch:8.2.4520: using wrong highlight for cursor line number

Problem:    Using wrong highlight for cursor line number.
Solution:   Take filler lines into account when using CursorLineNr.
            (closes vim/vim#9897)
127969cf98

src/nvim/screen.c

@@ -2890,9 +2890,9 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool noc
             } else if (wp->w_p_cul
                        && lnum == wp->w_cursor.lnum
                        && (wp->w_p_culopt_flags & CULOPT_NBR)
-                       && (row == startrow
+                       && (row == startrow + filler_lines
-                           || wp->w_p_culopt_flags & CULOPT_LINE)
+                           || (row > startrow + filler_lines
-                       && filler_todo == 0) {
+                               && wp->w_p_culopt_flags & CULOPT_LINE))) {
               // When 'cursorline' is set highlight the line number of
               // the current line differently.
               // When 'cursorlineopt' has "screenline" only highlight

src/nvim/testdir/test_diffmode.vim

@@ -1017,6 +1017,32 @@ func Test_diff_with_cursorline()
   call delete('Xtest_diff_cursorline')
 endfunc
 
+func Test_diff_with_cursorline_number()
+  CheckScreendump
+
+  let lines =<< trim END
+      hi CursorLine ctermbg=red ctermfg=white
+      hi CursorLineNr ctermbg=white ctermfg=black cterm=underline
+      set cursorline number
+      call setline(1, ["baz", "foo", "foo", "bar"])
+      2
+      vnew
+      call setline(1, ["foo", "foo", "bar"])
+      windo diffthis
+      1wincmd w
+  END
+  call writefile(lines, 'Xtest_diff_cursorline_number')
+  let buf = RunVimInTerminal('-S Xtest_diff_cursorline_number', {})
+
+  call VerifyScreenDump(buf, 'Test_diff_with_cursorline_number_01', {})
+  call term_sendkeys(buf, ":set cursorlineopt=number\r")
+  call VerifyScreenDump(buf, 'Test_diff_with_cursorline_number_02', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtest_diff_cursorline_number')
+endfunc
+
 func Test_diff_with_cursorline_breakindent()
   CheckScreendump