Онлайн видео на UltraWide мониторах (21:9).

Keep in mind that Flash Player itself isn't a video player.  We're a language runtime.  Developers write video players on top of the language platform that we provide.

There's not anything you can do about this from Flash Player.  You could capture the video and play it back in a dedicated video player like VLC, which would give you more flexibility in playback options.

In order to get a full-screen video on a 21:9 (2.3:1) monitor, you would need 21:9 video source.  The common video formats are all the same height (16:9, 4:3), so you're constrained vertically when scaling the video up to the monitor.

If you show the video at full height, then you're going to get bars on the side, because the video simply isn't wide enough to fill the available space on the monitor.  This is generally the best option, because you get to see the entire video.

The other option is to fill the screen horizontally, but this means that the video will be much taller than the space available on the monitor, so you would effectively crop the top and bottom sections of the video.  This can be problematic, as you may lose important things happening on the screen (like the tops of heads).  Because of this, we will generally fit vertically.

Since Flash Player isn't a video player, it's possible that the developer of that particular video player could implement the kind of video scaling that you're looking for, but nobody ever does in practice.