Well I guess I solved it with Gemini:
*If this is over kill let me know.
/* YouTube/video player fix */
.video-placeholder-container,
.video-placeholder-wrapper,
.lazy-video-wrapper {
/* This is the shield: it locks the player UI to 690px */
max-width: 690px !important;
width: 100% !important;
}
.lazy-video-wrapper iframe,
.lazy-video-wrapper video {
/* This fills the shield */
width: 100% !important;
max-width: 100% !important;
height: auto !important;
aspect-ratio: 16 / 9 !important;
margin: 0 !important;
}
/* Specifically target the uploaded video player container */
.video-container {
max-width: 690px !important;
text-align: left !important;
}