Custom AdsLoader: no supported way to play a skipped-over midroll without breaking AdPlaybackState invariants #3424

Media3 version

• 1.11.x (AdsMediaSource / AdPlaybackState)

Desired behaviour (force-play the last missed ad break after a seek)

Cues at 10 / 20 / 30 / 40 min. User starts at 5 min, seeks to 35 min. We want to play the last skipped break (30 min), resume at 35 min, keep 10/20 rewindable, and play 40 min once on the normal AdsLoader path.

Wanted:

• [10] [20] still playable if user seeks back
• [30] played after the seek
• [40] untouched, AdsMediaSource when playhead reaches it

How we build AdPlaybackState today

Media3 1.6+ (checkValidAdPlaybackStateUpdate): timeUs immutable, groups append-only, times must stay sorted (getAdGroupIndexForPositionUs walks from the last index). count = UNSET at the playhead stalls forever; count = 0 is a skipped placeholder that may grow (0 → N + URIs).
At start we publish one group per cue, sorted, at the real times :

• Cold start (playhead = 0)

  • t=0 preroll count = UNSET → AdsMediaSource waits, we resolve VAST then fill or skip
  • t=10m midroll count = 0 → content plays through
  • t=20m midroll count = 0
  • t=30m midroll count = 0
  • t=40m midroll count = 0

• Continue watching (bookmark = 12 min)

  • t=0 skipped / unused preroll at 0
  • t=10m midroll count = 0
  • t=12m resume preroll slot count = UNSET ← bookmark is in the initial sorted list
  • t=20m midroll count = 0
  • t=30m midroll count = 0
  • t=40m midroll count = 0

• Midrolls: count = 0 from launch so content is not blocked; AdsMediaSource can createPeriod at the real cue.
• 3–10 s before a cue we resolve VAST and grow that placeholder (count + media). We never move timeUs.
• Resume: a preroll at t=0 is ignored behind the playhead. We put the bookmark in the first sorted AdPlaybackState(...) (not appended later — that would sit after 40 min and hide midrolls). That slot stays UNSET until VAST, then we fill or skip.

Natural playback works. Force-play the last missed ad break after a seek does not.

What we cannot do

1. Move the 30 min group to playhead + 1 after 5→35 ->crash.
2. Rewrite all midroll timeUs after the seek — forbidden.
   -> 40 min already exists → append at ~35 min inserts a time before 40 min → unsorted / wrong index
3. Insert a group at the playhead — same case, forbidden (shifts later indexes).

Question

Do you see a way to implement this on top of the model above (real cue times, midrolls count = 0, UNSET preroll at 0 or bookmark, fill in the preload window) without breaking checkValidAdPlaybackStateUpdate?
In particular:
• After a user seek, can we play the last skipped-over midroll and then resume at the seek position, while later cues (e.g. 40 min) stay on the normal AdsMediaSource path?
• Is anything planned in this area (seek vs behind-playhead groups)?

Thank you in advance for your help and for taking the time to respond.