Firefox nested editable focus proof needs a real nested contenteditable surface

What happened

The legacy Firefox row looked scary because Slate had an explicit runtime carve-out:

• if a nested editable gets focus in Firefox
• force focus back to the outer editor

The easy modern candidate was the current editable-voids example. That was the wrong proof surface.

It only exposed normal buttons and form controls. It did not expose a true nested contenteditable descendant.

That meant the row could not honestly be closed or omitted from that example.

What fixed it

Build a real current surface with:

1. an outer current EditableBlocks
2. a void block
3. a nested inner div contentEditable

Then prove the actual Firefox behavior:

• click the nested editable
• assert document.activeElement is still the outer editor root

That is the real modern equivalent of the old focus-bounce comment.

Why this matters

This row was a good example of fake omission pressure.

Without a real nested-editable surface, it looked like maybe the row was gone. Once the surface existed, the current runtime already passed.

So the right lesson is:

• do not omit a legacy browser row just because the current example set is too weak to expose it

Reusable rule

For legacy browser quirks tied to a specific DOM shape:

• build the missing DOM shape first
• only then decide proof vs omission
• if the current runtime already passes on the real shape, close the row and move on