ReviewMarginLayer draws the cards in a strip right of the page, at the height
of the line each belongs to, with a dashed leader running through the gap
under it. A balloon is pushed down by the one above (balloonStack, measured
per card, unit-tested) and the column belongs to its page, so it never runs
past the sheet. The strip is only reserved where the margin has something to
show; the smoke test guards a card's content against its box.
Steffen Becker
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