patch 8.2.1345: Redraw error when using visual block and scroll

Problem:    Redraw error when using visual block and scroll.
Solution:   Add check for w_topline. ( closes #6597)
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index ed111a5..1e2ed45 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -220,3 +220,26 @@
   call delete('Xmac.txt')
   call delete(filename)
 endfunc
+
+" Test for scrolling that modifies buffer during visual block
+func Test_visual_block_scroll()
+  CheckScreendump
+
+  let lines =<< trim END
+    source $VIMRUNTIME/plugin/matchparen.vim
+    set scrolloff=1
+    call setline(1, ['a', 'b', 'c', 'd', 'e', '', '{', '}', '{', 'f', 'g', '}'])
+    call cursor(5, 1)
+  END
+
+  let filename = 'Xvisualblockmodifiedscroll'
+  call writefile(lines, filename)
+
+  let buf = RunVimInTerminal('-S '.filename, #{rows: 7})
+  call term_sendkeys(buf, "V\<C-D>\<C-D>")
+
+  call VerifyScreenDump(buf, 'Test_display_visual_block_scroll', {})
+
+  call StopVimInTerminal(buf)
+  call delete(filename)
+endfunc