Support for EPUB FXL was implemented after EPUB reflowable in the current codebase, and it benefits from various improvements that could be ported to improve the user experience.

On an FXL, a lot of positioning is handled using transform: and translate3d() along with animations. This allows for a very smooth transition (thank you hardware acceleration) across pages, especially when swiping on mobile.

Transition between resources is currently where the current codebase would benefit the most from this approach, as it would allow the user to see both chapter at once when swiping.

In addition to this new approach for navigating between columns/resources, we also need to return more than one locator at a time to better reflect how Readium locators work (any given screen can contain one or more locators).