Overall, the app is slick on my M1 Mini. I prefer it to the browser since my NAS server has barely enough power for real-time transcoding. The window resizing is unexpected, but mostly works. I might have found an OS-level bug related to it. I have two displays hooked up, and I've noticed that while the windows resizes on either, the video only follows the size of the window if it's on the primary display. On the secondary display, the video stays at its native resolution, and just crops to the window, or has borders on the right and bottom. The Secondary display is HDMI on the mini. The Primary is an HDMI port on a thunderbolt 2 dock (on thunderbolt3 via apple adaptor).
Edit: Looking at it closer, the UI elements (buttons and play bar at top, program name and icon on the bottom) are obscured by the video. if the window is larger than the video, you can interact with elements you can see. but elements you can't see (which is all of them if the window is smaller than the video), it doesn't seem like it can be interacted with. My suspicion is that the video layer/polygon (visually accelerated video playback destination) changes depending upon the screen, and not automatically hooked up to size change and layering callbacks (stays default size, and always top-most in window). could be an OS or an app-level problem, but unexpected because iOS devices don't usually have more than one screen in this way.