This is similar to e8d5b8be (tdf#113714
vcl menu bar window: avoid flicker, 2019-05-20), except that was for the
menu bar window, and this is for the floating window opening from that
one.

(cherry picked from commit c04169c5)

Change-Id: Ib24f4999ad3e8cbbecc058368e9d112e106e9178
Reviewed-on: https://gerrit.libreoffice.org/72726
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>