Since .16 deployment.
Edit an Page NS page, e.g. https://en.wikisource.org/w/index.php?title=Page:The_Poems_of_John_Donne_-_1896_-_Volume_1.djvu/58&action=edit, in side-by-side mode (must start in that mode)
The viewer zoom/pan doesn't work at all.
It can made to work by flipping to horizontal and back.
It looks like this line added in c6f71a5399ebb2718aa86b2842d52e7bd207ed49 is not prepared for $imgContHorizontal to be undefined:
var $layoutParts = $imgContHorizontal.add( $editForm.find( '.prp-page-container' ) );
Probably (without being able to actually test it myself right now), this should be the following, so it doesn't explode when $imgContHorizontal is not defined:
var $layoutParts = $editForm.find( '.prp-page-container' ).add( $imgContHorizontal );