patch 9.0.2134: ml_get error when scrolling

Problem:  ml_get error when scrolling after delete
Solution: mark topline to be validated in main_loop
          if it is larger than current buffers line
          count

reset_lnums() is called after e.g. TextChanged autocommands and it may
accidentally cause curwin->w_topline to become invalid, e.g. if the
autocommand has deleted some lines.

So verify that curwin->w_topline points to a valid line and if not, mark
the window to have w_topline recalculated in main_loop() in
update_topline() after reset_lnums() returns.

fixes: #13568
fixes: #13578

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_delete.vim b/src/testdir/test_delete.vim
index b05c27d..c00b8ca 100644
--- a/src/testdir/test_delete.vim
+++ b/src/testdir/test_delete.vim
@@ -1,6 +1,8 @@
 " Test for delete().
 
 source check.vim
+source term_util.vim
+source screendump.vim
 
 func Test_file_delete()
   split Xfdelfile
@@ -107,4 +109,25 @@
   call assert_fails('call delete(''foo'', 0)', 'E15:')
 endfunc
 
+" This should no longer trigger ml_get errors
+func Test_delete_ml_get_errors()
+  CheckRunVimInTerminal
+  let lines =<< trim END
+    set noshowcmd noruler scrolloff=0
+    source samples/matchparen.vim
+  END
+  call writefile(lines, 'XDelete_ml_get_error', 'D')
+  let buf = RunVimInTerminal('-S XDelete_ml_get_error samples/box.txt', #{rows: 10, wait_for_ruler: 0})
+  call TermWait(buf)
+  call term_sendkeys(buf, "249GV\<C-End>d")
+  call TermWait(buf)
+  " The following used to trigger ml_get errors
+  call term_sendkeys(buf, "\<PageUp>")
+  call TermWait(buf)
+  call term_sendkeys(buf, ":mess\<cr>")
+  call VerifyScreenDump(buf, 'Test_delete_ml_get_errors_1', {})
+  call term_sendkeys(buf, ":q!\<cr>")
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab