Version 2 includes a breaking change to the DOM structure of the Preview element.
In version 1, the .bp-navigate buttons were siblings with the .bp container div
<div class="bp" tabindex="0">
...
</div>
<button class="bp-btn-plain bp-navigate bp-navigate-left bp-is-hidden">
...
</button>
<button class="bp-btn-plain bp-navigate bp-navigate-right bp-is-hidden">
...
</button>
But in version 2, the buttons are now inside a new container div .bp-content.
<div class="bp" tabindex="0">
<div class="bp-content">
<button class="bp-btn-plain bp-navigate bp-navigate-left bp-is-hidden">
...
</button>
<button class="bp-btn-plain bp-navigate bp-navigate-right bp-is-hidden">
...
</button>
</div>
</div>
.bp-content is also the new point in which the various viewers will be dynamically inserted as children, i.e. .bp-doc, .bp-image, etc...
This change in structure is to account for the new thumbnails sidebar which will appear to the left of the viewer content.