YouTube caps an embedded player's max quality to the iframe's OWN inner viewport
size, so the small windowed player was stuck at ~360p (manual HD selection snapped
back); only fullscreen unlocked 1080p. Render the player at a fixed 1920x1080
logical size and CSS transform: scale() it down to fit the stage — the transform
doesn't change the iframe's window.innerWidth, so YouTube keeps seeing a 1080p
viewport and lets you pick 1080p while we display it small. A ResizeObserver keeps
the scale fitting the stage in both windowed and fullscreen.
Also move wheel-to-volume from the small centre overlay to the whole modal, so
scrolling anywhere over the player window adjusts volume.
peter
d6ce4a0477