DRAFT: (FEAT) implement progressive tile loading to reduce white-screen time Open

[grab-liyanjin]

Summary

This PR implements two-phase (progressive) tile loading to reduce white-screen
time when panning or zooming to unloaded areas.

Previously, a tile remained invisible (renderable=false) until all layout
work completed — including network fetches for glyphs/images required by symbol
layers (text labels, icons). On slow networks or cold cache this could leave
large blank areas for several seconds, even though fill/line/circle geometry was
already parsed and ready to draw.

After this change, geometry buckets are emitted to the render thread
immediately after parse() completes. Symbol buckets follow in a second
onLayout call once all glyph/image dependencies are resolved.

Demo

before	after
normal_loading.mp4	progressive_loading.mp4

How it works

GeometryTileWorker

• Adds a comingFromParse boolean flag (reset after each finalizeLayout()
  call).
• parse() sets comingFromParse = true before calling finalizeLayout().
• finalizeLayout() gains an early-exit path: when dependencies are still
  pending and firstLoad && comingFromParse && !renderData.empty(), it
  copies the already-populated renderData (geometry-only buckets, cheap
  shared_ptr copy) and invokes GeometryTile::onLayout with
  isPartialResult=true. This partial result is skipped when renderData is
  empty (symbol-only tilesets) to avoid marking a tile as renderable with
  nothing to show.

GeometryTile

• onLayout() gains a bool isPartialResult = false parameter.
• For partial results: sets loaded = true and renderable = true so the tile
  appears on screen immediately, but does not clear pending or fire the
  EndParse event — the tile stays pending until the full symbol result arrives.
• The full result (isPartialResult=false) replaces layoutResult normally and
  clears pending as before.

Behaviour unchanged for

• Non-first loads (reload / style change) — firstLoad is false, progressive
  path is skipped entirely.
• Tiles with no pending symbol dependencies — hasPendingDependencies() returns
  false, existing code path taken unchanged.
• Symbol-only tiles — renderData.empty() guard prevents an empty partial
  result from being sent.
• Still-mode rendering — correlation ID handling is unchanged; pending is only
  cleared on the full result.

Testing

Verified on Android (arm64) with a style containing road labels and POI icons:
geometry (roads, water, buildings) appears within the first render frame after
tile data arrives; text and icons fade in once glyphs are downloaded.

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.9% +1.59Mi  +1.3%  +442Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4253-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +51% +59.1Mi  +467% +27.9Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4253-compared-to-legacy.txt

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +2.68Ki  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-4253-compared-to-main.txt