DesktopMonotonicClock

The desktop JVM monotonic clock: System.nanoTime(), the base every deadline DesktopAudioSink publishes is computed on.

NOT System.currentTimeMillis(): that is wall time, it jumps when NTP or the user moves the clock, and a backwards jump makes the engine believe a frame it already showed is due again. System.nanoTime() is the JVM's monotonic source on all three desktop hosts (mach_absolute_time on macOS, CLOCK_MONOTONIC on Linux, QueryPerformanceCounter on Windows).

This pairing is the whole point of the object. javax.sound.sampled publishes no presentation timestamp of its own, so the sink builds every deadline as "now plus the audio still queued", and its "now" is this call. Measure the rest of the engine from any other source and audio and video sit at a constant offset no correction can find, because both sides believe they are right. DesktopOutputBackend pairs this clock with the sink so the mismatch cannot be assembled, exactly as the Android and Apple backends do.