Determine the current layout mode

It’s possible to determine the current LayoutMode. NutrientViewer.ViewState#layoutMode has three possible values: SINGLE, DOUBLE, or AUTO. You can query the current value from the ViewState at any time.

In the case of NutrientViewer.LayoutMode.AUTO, you can query the DOM to determine if a single page or two pages are being rendered per spread:

const firstSpread = instance.contentDocument.querySelector(
  ".PSPDFKit-Spread"
);
const count = firstSpread.querySelectorAll(".PSPDFKit-Page").length;

This has been tested on Nutrient Web SDK 2021.5.2.