Interesting, @lidja - I hate editing my posts on Cake on an iPad,( although I do edit them a lot since my typing and spell check are conspiring against me ) and on an iPad I see the black and the blue boxes like in @apm's post above - BUT - when I unlock the screen orientation and turn the screen to landscape mode, I ONLY see the blue box. So YES, I think orientation contributes in some wierd way with the editing parameters but only in portrait mode for some reason.
I do not see the black box when editing on my Mac, so I think the lesson is that when editing on an iPhone or iPad, turn the device to landscape mode. I DO NOT see the black box when editing on my Mac with its monitor ( but it is in landscape mode of course )
Interesting, I learned another new tip today! Thank you Cake and @lidja and @apm 😎