pdfDefaultRetainedSceneBytes function

int pdfDefaultRetainedSceneBytes({ PdfPerformancePlatform? platform, })

The default byte budget for retained page scenes (PdfPagePreviewCache's retained-scene LRU) on this platform.

A retained scene is a page the viewer can put back on screen with no record, no replay and no worker round trip - the cheapest possible scroll-back - and it is priced at the raster it stands in for (PdfPagePreviewCache.priceRetainedScene), so these numbers convert directly into pages: an ordinary letter page at fit width is ~8 MB, so web keeps about four of them and desktop about sixteen.

What it buys, and what it costs

Measured on a 40-page text report (tool/perf.sh web read-text / wheel-text), the tier is worth exactly what it retains: with it off, revisiting a page costs a full record + replay again (readBackSettleP50 ~220 ms, 4% of arrivals already sharp); with it on, every revisit inside the budget is free (0.03 ms, 100%). It is NOT what makes scrolling forward smooth - that is the render scheduler's motion-safe lane, which costs no memory at all.

The cost is real and close to the budget: this tier measured as ~97 MB of extra browser-agent memory at 64 MB on that document, because what sits around it (preview LoDs, live page state) grows with it.

Why not simply smaller

A budget under the warm window is worse than a small one - it is wasted work. The speculative warm records and builds scenes several pages either side of the reader; if the tier cannot hold that window, those scenes are evicted before anybody arrives on them and the warm pays for nothing. Measured at 32 MB on web (about four letter pages against a warm window of seven), tab memory fell to +25 MB but forward arrivals lost their warm entirely: readFirstSettleP50 went from ~0 ms back to ~180 ms, while scroll-back stayed free. So the floor here is "the warm window", not a round number.

Desktop keeps a chapter's worth - the machine has tens of GB. Web and mobile keep the warm window and no more. A host that wants the smaller footprint should lower the warm radius with it rather than this alone; the app's Auto memory mode can move both, and the process-wide PdfCacheRegistry trims every registered cache under a coordinated ceiling.

Implementation

int pdfDefaultRetainedSceneBytes({PdfPerformancePlatform? platform}) {
  const mb = 1024 * 1024;
  return switch (platform ?? detectedPdfPerformancePlatform) {
    PdfPerformancePlatform.desktop => 128 * mb,
    PdfPerformancePlatform.mobile => 64 * mb,
    PdfPerformancePlatform.web => 64 * mb,
    PdfPerformancePlatform.other => 64 * mb,
  };
}