Xwayland now has support for emulating resolution changes requested by an app through the randr or vidmode extensions. If a client makes a resolution change requests this is remembered and if the client then creates a window located at the monitor's origin and sized to exactly that resolution, then Xwayland will ask the compositor to scale it to fill the entire monitor.
For apps which use _NET_WM_FULLLSCREEN (e.g. SDL2, SFML or OGRE based apps) to go fullscreen some help from the compositor is necessary. This is currently implemented in mutter. If you are a developer of another compositor and have questions about this, please drop me an email.
I failed to get this all upstream in time for Fedora 31 final. But now it is all upstream, so 've backported the changes and created an update with the changes. This update is currently in updates-testing, to install this update run the following command:
sudo dnf upgrade --enablerepo=updates-testing --advisory=FEDORA-2019-103a594d07 