AsyncRendering property

AsyncRendering is the visual half of the async render pattern: it tells the single-page paint path to draw a small loading indicator (configured through LoadingText and LoadingColor) over the rendered page area while a caller-owned worker renders the current page. The default is False so existing applications upgrade with no visual change.

AsyncRendering does not push rendering onto a worker thread by itself. Since v1.37.0, TPdf serializes render and page-unload calls per instance, so a worker can render through the same component without overlapping PDFium render calls. The caller still owns worker dispatch, bitmap handoff, and document lifetime: do not mutate or unload the document while a render is in flight. Use TPdfFuture<T> from FPdfAsync to drive the worker, and let AsyncRendering / LoadingText / LoadingColor handle the visual affordance during the in-flight render.

The loading overlay is a small rectangle drawn in the center of the page area, sized to the LoadingText caption. Applications that need a different placement, animation, or progress percentage should keep AsyncRendering off and handle the overlay through OnPaint directly.

• The overlay only fires from the single-page paint path; two-page spread modes (dmTwoPageContinuous, dmTwoPageContinuousWithCover) currently do not show the indicator. Wire your own OnPaint if both modes need it.
• Without a worker doing actual off-thread work, AsyncRendering still draws the overlay during a sync render — useful for very high-DPI exports on a slow machine, but most callers will combine it with TPdfFuture.
• The render path that backs RenderPage / RenderTile is zero-copy when the destination DIB can be wrapped safely, and falls back to a copied buffer for bottom-up DIB layouts.