Chart Controls: Screenshot button does not work in Firefox

[edd]

Presumably Firefox doesn't support OffscreenCanvas? I have not been to check

Quick fix

• Hide :camera: if browser does not support the required APIs to mash canvasses in to a PNG

Better fix

• ~Polyfill?~ Use a non-visible Canvas rather than an OffscreenCanvas (as per @johnwalley's suggestion)

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