We (eventually - albeit possibly infinitely far in the future)
expect a timeline update with a window of known duration. This
also stops live radio stream playbacks transitioning to ended
state when their tracks are disabled.
As part of this fix, I found an issue where getPeriodPosition
could return null even when defaultPositionProjectionUs is 0,
which is not as documented.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176492024
856c2f8d3e