Fix a few inconsistencies between the media element spec and implementations

[Zaggy1024]

This fixes a few inconsistencies between the spec and browsers' implementations of a few behaviors:

1. Media elements' seek steps do not set the official playback position before parallel steps begin #11773: A script is not immediately guaranteed to receive a playback position on the media timeline after setting currentTime:

   console.log(video.duration); // 10
   let x = video.currentTime = 100;
   console.log(x); // 100
   console.log(video.currentTime); // spec: 100, implementations: 10

   The fix for this issue is to specify that the currentTime setter returns the value passed to it, and that the seek steps should synchronously set the official playback position to the chosen new playback position after clamping it to the earliest possible position, the duration, and the seekable ranges. This also ensures that currentTime reflects the new playback time after calling fastSeek(), which matches Firefox and Safari behavior.

2. Media elements can loop if they reach the end of media playback while paused #11774: Seeking to the end of a video while paused loops to the start:

   video.loop = true;
   video.currentTime = video.duration;
   setTimeout(() => console.log(video.currentTime), 250); // spec: 0, implementations: 10

   In order to fix this, when reaching the end of playback while the loop attribute is set, the seek to the earliest position is only run when not paused. In order to allow playback to restart when that condition does not pass, the internal play steps must then seek to the start if looping was skipped due to being paused the last time they were run.

3. Media element is not considered to have ended playback if loop is enabled after reaching the end #11775: Finally, if the loop attribute is set after playback has ended:

   video.loop = false;
   video.play();
   video.currentTime = video.duration;
   setTimeout(() => {
       console.log(video.ended); // true
       video.loop = true;
       console.log(video.ended); // spec: false, implementations: true
   }, 250);

   This is solved by specifying that playback has ended when the loop attribute was specified the last time that the end of playback was reached, meaning that modifying it before playback ends again does not affect the value of the ended attribute. This also allows playback to restart from the beginning even after specifying loop while playback is ended.

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• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

[zcorpan]

For (1), doesn't this

let x = video.currentTime = 100;
console.log(x); // 100

follow from JS semantics, regardless of what the setter does (so long as it doesn't throw)?

[Zaggy1024]

I'm sure you're right, I haven't worked closely with the JS spec. In that case, the change to specify the the value of the assignment is unnecessary, but I believe the point still stands that the playback position on the timeline is not guaranteed to be immediately observable by a script after seeking begins, with the parallel nature of the steps as they stand currently.

I'm not able to update the PR at the moment, but when I'm back home I'll remove the first part of that change.

[Zaggy1024]

Just remembered to get back to this, I've removed the part defining the result of the assignment. Setting the official playback position is still moved within the seek algorithm, though, as that result needs to clamp to the seekable regions in order to match the implementations.

[Zaggy1024]

I've updated the first commit to fix another bug where the official playback position would revert to the last available position if the media element is paused in the middle of a seek. To do so, the pause steps now skip setting the official playback position if a seek is in progress.