title_body: Resize readium document if content body has changed
upvoted_answer: I don't have much ability to answer your question (especially without seeing your code or knowing the required space in the viewport to hide or display). 
  
  I'm aware of and have occasionally tested things on Readium. But I never had a use case for supporting Readium for epubs. 
  
  If you support  ibooks and ADE, that's covering a lot of ground. I assume it means you have also tested on Google Play Books. 
  
  Update: I see that Bluefire has released an ios reader based on readium. So that counts for something. That is still a sliver of reading audience. 
  
  You said this is related to resizing. I have noticed that some apps have resizing flakiness. But resizing is something you usually do on a PC browser, not a mobile device. I would worry about whether this issue shows up on mobile devices. 
  
  Let me mention that Google Play Books also has  problems with regard to css related to page size. See my previous question here: How well does Google Play Books support page-oriented css? (i.e., page breaks) ).   If Readium is based on Chrome code base, I have to wonder if this issue is rooted in Chrome instead of Readium or GPB.