focusDistance property
Distance in pages from the focused page (0 = the page the viewport is on). Off-focus neighbours are prefetched, so their embedded images are decoded and shipped at a reduced resolution (prefetchImagePixelRatioFactor) to keep record traffic down (a Flate raster underlay ships as full RGBA - a single page can reach tens of MB, starving the visible page, #451). The page re-renders at full image resolution the moment a meaningful share of it becomes visible (see qualityVisible).
Focus is a ranking, not a visibility test - see onScreen, which is what decides whether this page is treated as a prefetch neighbour at all.
Implementation
final int focusDistance;