Marxian, thanks for adding the info mode!
This is great and fine for me, but I would prefer it like this:

1) Streaming
--> Search for a video
--> Go to detail view
--> Tapping thumbnail starts video preview in thumbnail (preview) size
--> An additional button below thumbnail would start fullscreen playback or switch to fullscreen playback when playback in preview size was already started.
--> From fullscreen a button like the "i" button would go back to detail view with video still playing in preview size
(ideally there would even be an option to keep video playing while going on browsing from detail view...e.g. a once started video can always be brought back by swiping up from bottom like downloads by swiping down )

2) Archive
--> Select file from archive
--> go to detail view playing in audio mode / preview mode and loading related comments / related videos from youtube
--> For videos, additional button below thumbnail / preview playback for switching to fullscreen
--> From fullscreen a button like the "i" button would go back to detail view with video still playing in preview size
(Long press in archive goes to fullscreen immediately)

Hope you get what I mean and don´t know if this is possible / feasible, but I think I would like it this way