THE FOLLOWING WORKS FOR GETTING RID OF WHITE VIDEO PLANE FLASH AT BEGINNING OF PLAYBACK:
I'm using Vuforia 9.3.3 with Unity 2019.3.Of6 and the following works:
I'm using WebM video file with transparency, VP8 Codec.
If you update the DefaultTrackableEventHandler, it NOW gets defaulted back to the original code every time you close and re-open Unity!!!
Therefore, you need to select settings in the ImageTarget's Inspector as follows.... (SEE ATTACHED IMAGE):
On the ImageTarget in the Inspector I set the DefaultTrackableEventHandler settings to:
Consider target as visible if it's status is: TRACKED
OnTargetFound: (1 event)
Runtime Only / Video_plane / VideoPlayer.Play
OnTargetLost (3 events)
Runtime Only / Video_plane / VideoPlayer Stop
Runtime Only / Video_plane / VideoPlayer Play
Runtime Only / Video_plane / VideoPlayer Pause
This got rid of the white flash of the blank of the video plane at beginning of video playback in the scene.
OnTargetLost appears to apply to before the target is ever found.