The handle mixed the hitbox-local drag origin with window-space
drag-move positions, so the first move snapped the ratio to a clamp
boundary (a panel suddenly grew and further drags did nothing). Drags
are now incremental window-space deltas applied to the freshly-read
ratio; drag-move routing is filtered to the owning split container so
nested splits no longer double-apply with wrong geometry; pair extent
excludes the divider itself so the boundary tracks the pointer 1:1.