There is a well known issue with html with 100% height and the WordPress (tinymce) visual editor that grows when loading and with each UI interaction.
In sage 8.x I was able to fix it by applying 100% height to body when it is of a page (body.page), the 100% height of html then hadn’t caused the issue anymore - or this approach I just found: Theme CSS affecting TinyMCE-editor behavior - #2 by cfx
However, in sage 9.x apparently this isn’t enough, the visual editor still grows.
I could enqueue extra styles only for admin and override the height 100% of html -
but I’d rather like to find a more elegant and simpler workaround instead.