[webvtt] Should line:100% render the cue offscreen? (#482)

jonknowles has just created a new issue for https://github.com/w3c/webvtt:

== Should line:100% render the cue offscreen? ==
The spec says that **line** ["offsets the cue box from the top, the right or left of the video viewport"](https://www.w3.org/TR/webvtt1/#webvtt-cue-line). I understand this to mean that for horizontal cues, the top of the cue box is offset from the top of the viewport in proportion to the percentage value of the line setting (or by some factor of line-height for integer line settings). So a line:0% would be flush to the top of the viewport. If that is so, it seems like line:100% cues should be offset all the way offscreen at the bottom of the viewport, like so: 
```
 ____________________________________________________________
|                         line:0%                            |
|                         line:10%                           |
|                         line:20%                           |
|                         line:30%                           |
|                         line:40%                           |
|                         line:50%                           |
|                         line:60%                           |
|                         line:70%                           |
|                         line:80%                           |
|_________________________line:90%___________________________| 
                          line:100%   
```

I noticed that Chrome will keep line:100% cues on screen, but Firefox lets them go offscreen. The videojs/vtt.js implementation seems to have logic to nudge the line:100% cue back onto the viewport here: https://github.com/videojs/vtt.js/blob/master/lib/process/move-box-to-line-position.js#L14:L40

But I don't see this behavior defined in the spec (maybe I'm overlooking something). I Is it expected that line:100% cues will be rendered offscreen?  

Please view or discuss this issue at https://github.com/w3c/webvtt/issues/482 using your GitHub account

Received on Wednesday, 19 February 2020 16:16:15 UTC