vim-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 vim/vim#6597)
f8992d47cd

src/nvim/screen.c

@@ -899,11 +899,12 @@ static void win_update(win_T *wp)
        || type == INVERTED || type == INVERTED_ALL)
       && !wp->w_botfill && !wp->w_old_botfill
       ) {
-    if (mod_top != 0 && wp->w_topline == mod_top) {
-      /*
-       * w_topline is the first changed line, the scrolling will be done
-       * further down.
-       */
+    if (mod_top != 0
+        && wp->w_topline == mod_top
+        && (!wp->w_lines[0].wl_valid
+            || wp->w_topline == wp->w_lines[0].wl_lnum)) {
+      // w_topline is the first changed line and window is not scrolled,
+      // the scrolling from changed lines will be done further down.
     } else if (wp->w_lines[0].wl_valid
                && (wp->w_topline < wp->w_lines[0].wl_lnum
                    || (wp->w_topline == wp->w_lines[0].wl_lnum

src/nvim/testdir/test_display.vim

@@ -184,3 +184,27 @@ func Test_scroll_CursorLineNr_update()
   call StopVimInTerminal(buf)
   call delete(filename)
 endfunc
+
+" Test for scrolling that modifies buffer during visual block
+func Test_visual_block_scroll()
+  " See test/functional/legacy/visual_mode_spec.lua
+  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