How does Angular's incremental hydration eliminate SSR flicker compared to full hydration in 2025 apps?

// app.config.ts
import { provideClientHydration, withIncrementalHydration } from '@angular/platform-browser';

export const appConfig = {
  providers: [
    provideClientHydration(withIncrementalHydration()),
    provideRouter(routes)
  ]
};

// Template: Critical hydrates instantly, rest on-demand
<button hydrate-priority="high" (click)="search()">Search</button>

@defer (hydrate on viewport) {
  <heavy-analytics-chart [data]="metrics"></heavy-analytics-chart>
} @placeholder {
  <skeleton height="300px"></skeleton>
} @loading (minimum 800ms) {
  <progress-spinner></progress-spinner>
}