[Meta] Playback bugs when changing speed

[mesvam]

mpv has a variety of subtle issues with when playback speed is changed, including A/V desync, seeking-like behavior, audio and/or video stuttering, etc.

Apparently, this may be a bit complicated to fix in a way that solves all of the issues simultaneously without other side effects, which will take some time, so I am creating this meta issue to track further progress here

Ideally, changing playback speed

• should be instantaneous and take effect immediately on both the video and audio streams
• should keep audio and video in sync at all times
• should not trigger a seek or cause seeking-like behavior
• should not introduce gaps or cause stuttering in video or audio
• should report correct values for time-pos and audio-pts at all times

See also

• Bug: Playback seeks backwards when speed is increased, and seeks forwards when speed is decreased #13513
• Increasing playback speeds and seeking during high speeds causes high av desynchronisation and gaps in audio #13613
• --untimed performance regression since commit e3af545 #13622
• more playback speed changes #13614