Task breakdown for #7510

• If video is playing, scrolling down the page minimizes the video to the corner.
• When scrolling back to top, video is maximized again.
• When video is minimized, swiping to the right dismisses the video (e.g. pause and moves it back to the top).
• Minimized video should be 'movable' by the user.

Format for docking videos

• Available only on video players that implement the video-interface.
• dock=(when-playing|never)
• Defaults to when-playing (this makes this an opt-out!)
• Possibly to support when-playing-or-autoplaying later. We are not sure if we want to support this for auto playing videos.
• Value will be ignored in A4A format

UX for docking videos

• Videos that are played by the user are minimized and docked to the top corner when about to go out of viewport as user scrolls.
• Player's builtin controls get hidden
• Swipe to the left dismisses the video (i.e. pause and dock back to original place)
• Tap expands the video to full width but (still docked to the top) and enables the player's builtin controls so user can take actions such as volume change, fullscreen, etc.
• Playing another video will dismiss the existing docked video.