WKWebView runs no service worker for a self-hosted server, so on iOS nothing
kept a map tile once it scrolled away. Binaries with the tileCache feature
answer a trek-tile:// scheme from a store on disk: a tile viewed once is
served from there for up to 30 days (Mapbox's limit for a device cache,
applied to every provider), then fetched anew; the store keeps under 250 MB,
dropping the least recently used tiles first, sends a unique app User-Agent,
and never prefetches. Mapbox's own resources (style, tiles, glyphs, sprites)
and Leaflet's raster tiles go through it, telemetry never does.
Offline, a Mapbox map in such an app now stays Mapbox where it was viewed,
and the pill offers the stored trip region instead. The offline settings
show what the store takes against its limit, with a way to empty it.
Offline, a Mapbox map, another OpenFreeMap style or a raster template the
device cannot serve now switches to the stored OpenFreeMap basemap, light or
dark with the colour scheme, keeping every marker, route and track, with a
small Offline map pill. It switches back once the connection returns.
"Prepare for offline" stores that basemap's region for every trip, whatever
the provider, and both styles' sprites and glyphs, including the scripts a
trip in East or South Asia labels its places in. The vector prefetch keeps to
OpenFreeMap's download rules (four requests at a time, 50 MB per run), raster
tiles are no longer prefetched from OpenStreetMap's own servers, which their
policy forbids, nor inside the iOS app, which has no service worker to serve
them.
Maurice