The fit toggle relied on canvas.fit { max-width/max-height: 100% }.
The canvas is a flex item, and a flex item's automatic minimum size
(min-width/min-height: auto → the replaced element's intrinsic size)
overrides max-*, so an oversized desktop never shrank — it just got
centred and clipped, looking like the toggle did nothing.
Replace the CSS approach with explicit JS sizing: relayoutCanvas()
computes a scale factor (min of width/height ratios, capped at 1 so
it never upscales) and sets canvas.style.width/height in px,
preserving aspect ratio. Recomputed on fit toggle, window resize,
and framebuffer-size (ServerInit). Pointer mapping is unaffected —
sendPointer already derives scale from getBoundingClientRect().
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