2bf1531 fixed the reported symptom and introduced the opposite one: markers that
had been landing left came right, and then sat well ABOVE where they belonged.
It was the right diagnosis and the wrong remedy. The drawing is painted with
object-fit: contain inside a fixed 390x300 preview, so it is letterboxed on one
axis and a percentage of the BOX is not a percentage of the DRAWING. That part
was true. But the fix resized the frame to shrink-wrap the image, and the
centring transform above it translates by percentages - which resolve against
that same element. Making the frame image-sized silently broke the transform's
assumption that the frame is the box. Width still matched at 390, height no
longer did, which is exactly x-right and y-wrong.
The frame fills the box again and object-fit: contain is back. Where the bars
fall is now computed instead: paintedBox works out the fraction of the box the
drawing covers and the gap before it, per axis, and the marker maths folds that
in. The transform's assumption stays true because the element it translates
never changes size.
Checked per axis rather than asserted. A tall drawing pillarboxes, so x
compresses and offsets inward - a left edge at 2 percent of the drawing lands at
13.1 percent of the box, which is where the drawing actually starts - and y is
untouched. A wide drawing does the reverse. A level shaped like the box is a
no-op, so nothing that already looked right moves. Centre maps to centre on
every shape.
NEW COUPLING, deliberately noted at both ends: PREVIEW_WIDTH and PREVIEW_HEIGHT
must stay in step with .map-preview in the styles. Resize the preview without
touching them and this drifts back.
Still not confirmed against a live instance, and this is the second attempt at
this bug, so it wants eyes on three cases: one that was landing left, one that
was landing high, and one that was right all along.
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