renderHeadless
Synchronously renders an element description through the retained core and rasterizes its paint output.
Viewport and physical-size validation occurs before the description is validated or any node lifecycle hook runs. Render callbacks and temporary-tree cleanup run on the calling thread, and the temporary tree is always closed after creation. Semantics remain logical, unscaled, unclipped, unmodified, and in core emission order.
Return
an immutable frame containing the physical image and logical semantics.
Parameters
the caller-owned immutable root description.
the positive logical fixed viewport.
the positive integer logical-to-physical scale.
Throws
when the viewport or scale is invalid, or a command is unsupported or null from Java.
when checked physical dimensions, pixel area, or derived raster storage exceeds Int.MAX_VALUE.
when core work or cleanup fails; the exact work failure remains primary and distinct cleanup failures are suppressed once.