Video in Media, can no longer select video thumbnail for poster image from Media.

### Impacted plugin

Jetpack

### What

User is unable to see the video thumnail in large enough window, as selecting the thumbnail has changed.

### How

This was the original action: https://wordpress.com/support/videopress/#choose-a-thumbnail-in-media
User has to select poster image from tiny side window, instead of a full screen view.

Start by reproducing the VideoPress thumbnail-selection flow described in the linked support article and compare the current Media experience with the original action. Trace the UI entry point that opens the poster-image picker; done means the video thumbnail can again be selected from a sufficiently large view rather than only the tiny side window.

Written by the indexing model from the issue text.