patch 9.1.0815: "above" virtual text causes wrong 'colorcolumn' position

Problem:  "above" virtual text causes wrong 'colorcolumn' position.
          (@matrdr)
Solution: Use the number of cells instead of bytes for vcol_off_tp.
          (zeertzjq)

fixes: #15946
closes: #15948

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index bbb911f..bf561ca 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -3203,6 +3203,27 @@
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_prop_with_multibyte_above()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      setlocal number colorcolumn=10
+      call setline(1, ['11111111', '', '333333333', '', '55555555555'])
+
+      let vt = 'test'
+      call prop_type_add(vt, {'highlight': 'ToDo'})
+      for ln in range(1, line('$'))
+        call prop_add(ln, 0, {'type': vt, 'text': '…', 'text_align': 'above'})
+      endfor
+      normal G
+  END
+  call writefile(lines, 'XscriptPropMultibyteAbove', 'D')
+  let buf = RunVimInTerminal('-S XscriptPropMultibyteAbove', #{rows: 16, cols: 60})
+  call VerifyScreenDump(buf, 'Test_prop_multibyte_above_1', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_prop_with_multibyte_below()
   CheckRunVimInTerminal