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)

Author: brammool

src/drawscreen.c

@@ -1659,10 +1659,13 @@ win_update(win_T *wp)
 #endif
 	    )
     {
-	if (mod_top != 0 && wp->w_topline == mod_top)
+	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, the scrolling will be done
-	    // further down.
+	    // 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

src/testdir/dumps/Test_display_visual_block_scroll.dump

@@ -0,0 +1,7 @@
+|{+0&#e0e0e08| | +0&#ffffff0@72
+|}+0&#e0e0e08| | +0&#ffffff0@72
+|{+0&#e0e0e08| | +0&#ffffff0@72
+|f+0&#e0e0e08| | +0&#ffffff0@72
+>g| +0&#e0e0e08| +0&#ffffff0@72
+|}| @73
+|-+2&&@1| |V|I|S|U|A|L| |L|I|N|E| |-@1| +0&&@29|7| @8|1@1|,|1| @9|B|o|t| 

src/testdir/test_display.vim

@@ -220,3 +220,26 @@ func Test_unprintable_fileformats()
   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

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1345,
 /**/
     1344,
 /**/