#1853: Fix direction detection for display:contents lists

[Yahiro025]

Fixes #1853.

Problem. Sortable on a display: contents container inside a CSS grid (especially with an incomplete last row) can place the dragged item at the end of the grid. Setting direction: 'horizontal' or attaching Sortable to the grid itself worked around it.

Root cause. _detectDirection inspects the sortable’s own style. display: contents has no box; children participate in the ancestor’s grid/flex. Block children made the function return vertical. _ghostIsLast then treated “to the right of the item union” as “after the list.”

Fix. Walk past display: contents ancestors (including assignedSlot and a shadow host) and use that ancestor for flex/grid/width. Also treat inline-flex / inline-grid like flex / grid.

Tests. tests/display-contents-grid.html + direction assertions (grid/flex-row → horizontal; flex-col/block/1-col grid → vertical; nested contents and inline-grid → horizontal). Drag item 1 onto item 3 expects 2, 3, 1, 4, 5.

Verification. npm i && NODE_ENV=umd npx rollup -c ./scripts/umd-build.js && npx testcafe chrome:headless tests/Sortable.test.js --reporter spec → 20 passed. Builds not committed.

AI-assisted disclosure. Drafted with AI assistance. Root cause confirmed by detectDirection returning vertical on a contents+grid fixture before the walk-up.

Please merge only to master.

[Yahiro025]

Fixes direction detection for display: contents lists (#1853). Branch is merge-clean with no open threads — ready for review or merge whenever you have a chance.