Slate v2 destructive delete must clean empty leaves before render

Problem

Repeated destructive deletes through richtext leaf boundaries can leave empty marked/code leaves in a non-empty paragraph. If React renders those leaves as line-break zero-width nodes, the visible editor gains fake blank lines even when model text and selection appear correct.

Symptoms

• Repeated Backspace or Option-Backspace around the richtext <textarea>! segment created blank lines in the first paragraph.
• The first block model text could be coherent while innerText, textContent, and zero-width DOM nodes diverged.
• Follow-up typing still worked often enough that model-only tests missed the visual regression.

What Didn't Work

• Patching one richtext row was too narrow; the same class appears at mark, code, decoration, inline, range-delete, cut, and word-delete boundaries.
• Hiding the in React alone was the wrong owner. React can defend rendering, but the invalid committed model shape still exists.
• Treating legacy parity as "copy empty-leaf internals" weakens v2. Legacy tolerates more zero-width render shapes; v2 should classify or remove invalid leaves before render.

Solution

Make destructive leaf cleanup a core lifecycle owner, then prove the rendered DOM shape through slate-browser.

Core cleanup removes empty text leaves unless they are structurally required:

const requiredInlineSpacer = isRequiredInlineSpacer(editor, children, index)
const requiredEmptyBlockAnchor = !parentHasText && emptyTextChildren <= 1

if (requiredInlineSpacer || requiredEmptyBlockAnchor) {
  continue
}

maybeRebaseSelectionBeforeRemoval(editor, childPath, affinity)
removeNodes(editor, { at: childPath, voids: true })

React rendering keeps the same invariant from leaking visually:

empty block anchor -> one line-break placeholder
required inline spacer -> non-line-breaking sentinel
mark placeholder -> non-line-breaking sentinel
removable empty marked/code/decorated leaf -> must not reach render truth

slate-browser must assert rendered DOM shape in generated destructive gauntlets:

await editor.assert.renderedDOMShape({
  blockIndex: 0,
  innerText: firstBlockModelText,
  noUnexpectedZeroWidthBreaks: true,
  textContent: firstBlockModelText,
  zeroWidthBreakCount: 0,
})

Release proof should include the guard names:

• leaf-lifecycle-contract
• selection-rebase-contract
• rendered-dom-shape-contract
• destructive-leaf-boundary-gauntlet
• legacy-leaf-delete-parity

Why This Works

The root bug is not "Backspace timing" once the editing epoch is model-owned. The root bug is committed tree shape plus render shape:

destructive edit
  -> empty leaf survives
  -> React treats it as a line-break zero-width node
  -> non-empty paragraph gains fake visual lines

Owning the lifecycle in core prevents invalid empty leaves from becoming render truth. Keeping a React rendered-DOM contract catches custom render paths. Adding slate-browser DOM-shape assertions closes the exact gap that model text and selection tests missed.

Prevention

• Destructive-edit tests must assert model tree/text, model selection, visible DOM text, zero-width shape, DOM selection target, and follow-up typing.
• Non-empty blocks must not contain unexplained zero-width nodes.
• Empty leaves need a named lifecycle class: empty block anchor, inline spacer, mark placeholder, or temporary transaction anchor.
• Release proof must fail if the DOM-shape assertion path is removed.
• Legacy parity should classify copied, improved, and rejected behavior instead of silently copying old internals.

Related Issues

• Slate React repair-induced selectionchange must stay model-owned
• Slate browser Playwright helpers must normalize zero-width selection and wait for selection sync
• V2 editable text primitives should compose leaf text zero-width and placeholder