Fixes #16101
The simplest fix seems to be to just drop the sizePolicy property, as we don't use that on any other instances of QDialogButtonBox.
master (76e2cded477bc483ec610212bdadf21fe35292d4):

This PR:

utACK d595b4aae9d6eca7094ed5612f4773d98e422057
tACK d595b4aae9d6eca7094ed5612f4773d98e422057
utACK d595b4aae9d6eca7094ed5612f4773d98e422057