Firefox drag drop proof needs example-owned drop mutation and document-level drag cleanup

What happened

The first Firefox drag/drop row looked green on paper and red in reality.

We added generic drag handlers to Editable and a new drag-drop-cleanup example, then tried to reorder void cards with a plain Playwright dragTo.

The first attempt crashed during dragstart. After that fix, the crash disappeared, but the order still never changed.

That split the problem into two different truths:

• the generic drag lifecycle inside Editable mattered
• the proof row still needed an example that owned an actual drop mutation

Without the second part, the test was poking browser drag events but not proving the legacy Firefox case we cared about.

What fixed it

Two changes made the row honest:

1. Editable stopped routing dragstart back through findPath(node) and resolved the dragged path from the DOM node directly.
2. The example owned the reorder on drop:
   • onDragStart wrote a card id into dataTransfer
   • onDragOver allowed drop on the explicit zone paragraphs
   • onDrop replaced the top-level editor children to reorder the cards and called preventDefault()

That last detail is the point of the row. When the example short-circuits the internal drop handler, the document-level dragend / drop cleanup in Editable still has to clear internal drag state after the dragged node unmounts.

The proof is the second drag:

1. drag card A after card B
2. let the first drop unmount and remount the dragged structure
3. drag card A back before card B
4. assert both orders succeed in Firefox

Why this matters

Legacy Slate carried the global Firefox drag cleanup for a reason. Dragged nodes can disappear before their own local dragend path finishes.

If the current proof row only checks one drag, it can miss the dirty internal drag state that breaks the next interaction.

Reusable rule

For Firefox drag/drop parity rows:

• do not rely on a vague generic drag gesture alone
• make the example own a deterministic drop mutation
• keep the internal document-level dragend / drop cleanup active
• prove the row with two drags, not one