The line was hidden while a drop would leave the column where it is, so a drag
showed nothing until it crossed a neighbour's middle. It now shows from the
first movement, on the nearer edge of the column's own slot when the drop
would change nothing, and at the first boundary over the mark column. The
choice of boundary moves to a tested function in the columns module.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Gregory SchierandClaude Opus 5.5
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