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Shell-backed large-document selection must fail closed in DOM and run broad ops through the model

docs/solutions/performance-issues/2026-04-11-shell-backed-large-document-selection-must-fail-closed-in-dom-and-run-broad-ops-through-the-model.md

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Original Source

Shell-backed large-document selection must fail closed in DOM and run broad ops through the model

Problem

Once far islands stop mounting editable descendants, the browser no longer owns truth for whole-document selection.

That breaks the default assumptions behind Ctrl+A, selection sync, and paste.

Symptoms

  • Ctrl+A on a shell-backed document would otherwise select only the mounted editable DOM
  • DOM selection sync would keep trying to project a Slate range through unmounted islands
  • browser-default paste on a shell-backed selection would be wrong even if the model selection was right

What Didn't Work

  • treating broad ops as “just browser behavior” after introducing shells
  • continuing to call DOMBridge.toDOMRange(...) for selections that span unmounted islands
  • allowing paste to fall back to browser default when the mounted DOM no longer represents the full model selection

Solution

In editable.tsx:

  • intercept Ctrl+A in large-document mode
  • map it directly to the full-document model range
  • mark shell-backed selection state on the root
  • skip DOM range projection for shell-backed selections and clear native DOM selection instead
  • route paste through ReactEditor.insertData(...)
  • if selection is shell-backed, prevent browser default even when clipboard insertion fails, so the runtime fails closed instead of corrupting the wrong mounted subset

Why This Works

The moment the runtime stops mounting the whole editable tree, broad operations must stop trusting the browser to describe the whole document.

The browser still owns local live editing inside mounted islands. It no longer owns whole-document selection semantics.

So the runtime split has to become explicit:

  • mounted local edits: DOM truth
  • shell-backed broad ops: model truth

That keeps shells cheap while preserving correct editor behavior.

Prevention

  • any shell or occlusion layer must audit Ctrl+A, paste, and selection sync explicitly
  • if a selection spans unmounted content, do not project it back into DOM as if the whole tree still exists there
  • shell-backed broad ops should fail closed in DOM and stay model-driven
  • do not let browser-default paste touch a partially mounted document when the model selection is larger than the mounted DOM