YouTube picks an embed's quality at player-creation from the iframe's internal
viewport and latches it; a windowed-size embed was stuck ~360-480p, forcing a
manual fullscreen + gear trip to 1080p. Create the iframe at a fixed 1920x1080
logical size and CSS transform-scale it DOWN to fit (a CSS transform doesn't
shrink the internal viewport, so YouTube still serves 1080p, latched for the
session even while it paints small). A letterbox-contain fit via a
ResizeObserver handles both the windowed and big-view boxes. Gated behind
FORCE_HD_LOGICAL_SIZE for an easy revert if a restrictive network overrides it.
Also add a maximise (fullscreen) button to the windowed player's meta row: the
only fullscreen control reachable in the windowed video was YouTube's own, which
hides our chrome. It mirrors the F key and leads to the docked bar + our true
fullscreen. Corrects the stale code note that claimed the scale trick fails
(that was scaling a SMALL player up; scaling a LARGE player down works).
peter
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