The unrolled canvas put the finger's +x where the 3D default view puts screen-left,
and the 3D flow arrows rasterised as 0.23 px hairlines because their stroke was 2 %
of their own sub-millimetre length. The 2D presentation is now the derived half turn
(persisted, set from the plot-settings dialog) and the 3D stroke is one canvas pixel
— the width the 2D canvas draws — verified numerically and with real-GL snapshots.
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