I have just updated GenerateBlocks to the new release and when editing in the WP backend, if you switch to tablet, then mobile and then back to desktop the editing screen falls to bits.
I have tested this with older and newer (flex) sites on two different hosts and get the same result.
Sorry mate, I have just discovered this is not a GenerateBlocks issue, it’s either a WordPress issue or an issue with the Mac OS update that came out yesterday.
Just tried it in Firefox, Chrome and Safari and the same thing happens. For the record you are correct it does not do it at all when using the GenerateBlocks responsive controls.