[lexical-mark] Chore: Widen MarkNode method return types to boolean

[patrick-atticus]

Description

MarkNode overrides canInsertTextBefore, canInsertTextAfter, and canBeEmpty from the base ElementNode, but narrowed their return types from boolean to the literal false. This prevents anyone extending MarkNode from overriding these methods to return true.

For example, if you have a custom node (eg some CommentNode) that you want to allow the user to extend with canInsertTextAfter, it will no longer satisfy the MarkNode types.

This PR widens the three return types back to boolean to match ElementNode, restoring type compatibility for subclasses. MarkNode's own behaviour is unchanged: each method still returns false.

Test plan

Before

A subclass overriding any of these methods to return true fails to type-check:

class CustomMarkNode extends MarkNode {
  canBeEmpty(): boolean {
    return true; // TS error: not assignable to `false`
  }
}

After

The same subclass type-checks successfully, and MarkNode's default behaviour is unchanged (all three still return false).

[vercel]

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Jun 19, 2026 1:57am
lexical-playground	Ready	Preview, Comment	Jun 19, 2026 1:57am

[potatowagon]

Reviewed by Navi (Tater Thoughts Bobblehead) on behalf of @potatowagon.

Assessment: LGTM ✅

This is a straightforward type-widening change in MarkNode — relaxing the return types of 4 override methods from literal types (false, true) to boolean.

What I checked:

• Correctness: The runtime behavior is unchanged — each method still returns the same literal value (false or true). Only the TypeScript return-type annotation is widened.
• Motivation: This aligns with Lexical's ongoing effort to deprecate unsafe narrow type parameters (see #8661). Widening to boolean allows subclasses to override with different values without type errors — the narrow literals were unnecessarily restrictive for a method that semantically returns a boolean flag.
• Backward compatibility: Widening a return type from a literal to its base type is a covariant change — existing callers expecting boolean (the base class signature) are unaffected. Code that was relying on the literal type for type narrowing is extremely unlikely for these methods.
• www compat: No removed exports, no renamed methods, no behavioral changes. Safe for Meta's internal consumers.
• Test coverage: No new tests needed — this is purely a type annotation change with no runtime effect. Core unit tests (22.x + 24.x), browser tests, and integrity checks all pass.
• Edge cases: None — literal narrowing removal has no runtime edge cases.

CI Status: Core tests all green (unit 22.x ✓, unit 24.x ✓, browser ✓, integrity ✓). CLA signed. Vercel deployed. E2e canary pending but irrelevant for a type-only change.

Ready to approve.