=== Where getApplicationStartup() actually lives (Spring Framework 7.0.9) ===

Injecting ApplicationContext and calling getApplicationStartup() does not compile:

  [ERROR] .../config/StartupBeans.java:[22,45] cannot find symbol
    symbol:   method getApplicationStartup()
    location: variable context of type org.springframework.context.ApplicationContext

javap says the accessor is declared one interface down, and also on the bean factory:

  $ javap -cp . org.springframework.context.ConfigurableApplicationContext | grep ApplicationStartup
    public abstract void setApplicationStartup(org.springframework.core.metrics.ApplicationStartup);
    public abstract org.springframework.core.metrics.ApplicationStartup getApplicationStartup();

  $ javap -cp . org.springframework.beans.factory.config.ConfigurableBeanFactory | grep ApplicationStartup
    public abstract void setApplicationStartup(org.springframework.core.metrics.ApplicationStartup);
    public abstract org.springframework.core.metrics.ApplicationStartup getApplicationStartup();

=== The buffering API surface (spring-boot 4.1.1) ===

  $ javap -cp . org.springframework.boot.context.metrics.buffering.BufferingApplicationStartup
  public class BufferingApplicationStartup implements org.springframework.core.metrics.ApplicationStartup {
    public BufferingApplicationStartup(int);
    public void startRecording();
    public void addFilter(java.util.function.Predicate<org.springframework.core.metrics.StartupStep>);
    public org.springframework.core.metrics.StartupStep start(java.lang.String);
    public StartupTimeline getBufferedTimeline();
    public StartupTimeline drainBufferedTimeline();
  }

  $ javap -cp . org.springframework.boot.context.metrics.buffering.StartupTimeline$TimelineEvent
  public class StartupTimeline$TimelineEvent {
    public java.time.Instant getStartTime();
    public java.time.Instant getEndTime();
    public java.time.Duration getDuration();
    public org.springframework.core.metrics.StartupStep getStartupStep();
  }

Note: there is no setter for the buffer size and no Spring property that installs this.
The only constructor takes the capacity, and it must be handed to SpringApplication
before run().

=== You do not need a @Bean method for it ===

Declaring one produces a startup failure, because Boot has already registered the
instance as a singleton:

  Parameter 0 of constructor in com.ankurm.startup.web.StartupDiagnosticsEndpoint
  required a single bean, but 2 were found:
        - bufferingApplicationStartup: defined by method 'bufferingApplicationStartup'
          in class path resource [com/ankurm/startup/config/StartupBeans.class]
        - applicationStartup: a programmatically registered singleton

So the correct move is to inject ApplicationStartup directly and narrow with instanceof.
Injecting BufferingApplicationStartup by its concrete type compiles and works -- until
somebody runs without tracking, at which point the application will not start at all.