[openstreetmap/openstreetmap-website] Dashboard map under/overzooms map tiles at fractional zoom levels (Issue #6618)

[Minh Nguyễn]

1ec5 created an issue (openstreetmap/openstreetmap-website#6618)

#6504 migrated the map control on the Dashboard page from Leaflet to MapLibre GL JS. GL JS responds to zoom gestures incrementally, so it’s easy to end up at a fractional zoom level. At fractional zoom levels, it underzooms or overzooms raster tiles to fit. The OSM Carto maintainer points out in https://github.com/openstreetmap/openstreetmap-website/pull/6504#issuecomment-3656355664 that OSM Carto wasn’t designed to be underzoomed. Further work on #6234 would expose fractional zooming to more prominent maps that show other featured raster tile layers. Back when this site migrated to Leaflet, Leaflet didn’t support a fractional zooming option yet (#1153), so we never really considered this implication as far as I can tell.

The [`roundZoom`](https://maplibre.org/maplibre-gl-js/docs/API/classes/RasterTileSource/#roundzoom) option avoids underzooming but exacerbates overzooming: https://github.com/mapbox/mapbox-gl-js/issues/1030#issuecomment-76043779. It may also be possible to disable incremental zooming with the workaround in https://github.com/maplibre/maplibre-gl-js/issues/1591#issuecomment-2748319483, but I haven’t tried it to see if the user experience is similar to Leaflet with this feature disabled.

Off the top of my head, [geojson.io](https://github.com/mapbox/geojson.io/) and the [Valhalla FOSSGIS demo app](https://github.com/valhalla/web-app/) also use the default tile zoom strategy. I’m unaware of any other sites that use MapLibre to show raster basemap tiles with a different tile zoom strategy. On the desktop, QGIS consistently overzooms raster basemap tiles, including OSM Carto, but it also underscales the map on HiDPI displays: qgis/QGIS#63249.

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