Split from #1248: resizing the Xephyr window sort of works (as long as the patch is applied to Xephyr), but the geometry seems wrong - like the Xephyr window uses absolute coordinates instead of relative to the corral window.
The new "start-desktop" subcommand (#1247) means that we're much less likely to use Xephyr. And Xephyr needs patching already, so let's push this back.
Tried again on centos7 and Fedora and the Xephyr window is no longer resizable, so there's nothing to fix!
Related change: r16902.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1249