Gets or sets what receives the run's progress: the share of the run's content stream executed so far, from 0 to 1;
null, the default, for none.
public IProgress<double>? Progress { get; set; }
Reported on the running thread, synchronously, where the run checks CancellationToken (every 1,024 operators),
then once with exactly 1 when the run completes; a run that throws reports no completion. Values never decrease. The share is
measured in bytes of the top-level stream (for a page, its Contents joined, ISO 32000-2 §7.7.3.3 Table 31), so the
work of nested forms counts at the position of their Do operator.
The receiver may cancel CancellationToken's source; the run stops before its next operator. A host without threads (Blazor WebAssembly) can bound a build that way, by cancelling once its time slice is used.