[popcornmix]

Thanks @menakite, I'd not spotted code was needed there.

@FernetMenta We know when the Pi has finished it's part of the refresh rate change (it takes a few vsyncs and we do get a callback when that is done), but the TV may then take a number of seconds to stabilise, and there's nothing on the HDMI cable that shows when that is complete, so I don't think anything better can be done that using a GUI setting (or just living with losing the first few seconds).