hitTest

Maps a viewport point (the space gesture callbacks like onTap report in) to the page under it, or null when it lands on background/spacing.

Inverts the zoom/pan layer first (the layer scales around the viewport centre, then translates by panOffset, the same math setZoom's focal logic composes), locates the page slot from the geometry the layout reported, then maps display-space points through the inverse of KitePage.displayToDeviceBase into page space: PDF pages get user space (y-up, rotation unfolded), EPUB pages their document space.