patch 8.2.4497: wrong color for half of wide character next to pum scrollbar

Problem:    Wrong color for half of wide character next to pum scrollbar.
Solution:   Redraw the screen cell with the right color. (closes #9874)
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index cb64376..033bb37 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -606,6 +606,24 @@
   call delete('Xpreviewscript')
 endfunc
 
+func Test_scrollbar_on_wide_char()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, ['a', '            啊啊啊',
+                        \ '             哦哦哦',
+                        \ '              呃呃呃'])
+    call setline(5, range(10)->map({i, v -> 'aa' .. v .. 'bb'}))
+  END
+  call writefile(lines, 'Xwidescript')
+  let buf = RunVimInTerminal('-S Xwidescript', #{rows: 10})
+  call term_sendkeys(buf, "A\<C-N>")
+  call VerifyScreenDump(buf, 'Test_scrollbar_on_wide_char', {})
+
+  call StopVimInTerminal(buf)
+  call delete('Xwidescript')
+endfunc
+
 " Test for inserting the tag search pattern in insert mode
 func Test_ins_compl_tag_sft()
   call writefile([