The drag preview divided the edge height by the raw panel count on the target
side. Closed panels keep their stack slot, so a hidden panel (e.g. the block
palette tucked away while Properties stays open) split the ghost and the drop
index as if it were rendering — leaving a half-height preview on that edge.
Size slots from the panels actually rendered on the edge instead, via a shared
visibility helper used by both the drop-index math and the ghost/edge-tint
geometry.
drop_index previously mapped a pointer position inside a lone full-height panel
to a top/bottom insertion split, offering an impossible drop. Clamp to the
single slot when the edge holds one panel, and cover the multi-slot mapping with
tests.
Karim Jerbi