Fix: avoid infinite transform loop in Shiki/Prism highlight toggle

[messiawrq-design]

Description

Closes #8605

Fixes an issue where changing syntax highlighters (e.g. from Prism to Shiki) on a Code block inside the Playground causes an infinite transform error (errorOnInfiniteTransforms).
The root cause was that getIsSyntaxHighlightSupported() returns undefined initially or during switching phases when nodes are parsed, which made the ! negative condition evaluate to true, thus endlessly re-triggering $codeNodeTransform by calling setIsSyntaxHighlightSupported(true/false).

Implementation Details:

• Updated CodeHighlighterPrism.ts to strictly check !== true / !== false before mutating isSyntaxHighlightSupported.
• Applied identical strict conditional fixes to CodeHighlighterShiki.ts and FacadeShiki.ts.
• Validated fixes locally passing pnpm build and pnpm test.

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Jun 1, 2026 1:56am
lexical-playground	Ready	Preview, Comment	Jun 1, 2026 1:56am

[messiawrq-design]

I signed it!

[potatowagon]

Review from potatowagon's AI reviewer:

Thanks for the fix! The logic here is correct — using strict inequality checks to avoid the infinite transform loop is the right approach.

However, I'd recommend looking at #8606 by @levensta which addresses the same infinite loop issue with a slightly cleaner approach (setting the unsupported flag only once). Between the two PRs:

Concerns with this PR:

1. CLA not signed — the Facebook CLA check is failing, which blocks merge
2. Extraneous code.patch file — appears to be accidentally committed
3. Duplicate fix — [lexical-code-shiki] Bug Fix: Set the unsupported syntax flag only once #8606 solves the same problem and is further along in review

What I verified: The code logic is sound — the strict inequality checks prevent re-triggering transforms when the state is already set. No regressions to existing behavior.

Recommendation: Unless there's a reason to prefer this approach over #8606, I'd suggest the maintainers go with #8606 which doesn't have the CLA/cleanup issues.

[etrepum]

The solution in #8606 is better so this PR will be closed when that PR is merged

[potatowagon]

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

Concern ⚠️ — CLA check is failing; code fix looks correct but the PR includes a stale patch file.

What this does: Fixes an infinite transform loop in the Shiki/Prism code highlighters. The bug: getIsSyntaxHighlightSupported() can return null (not just true/false), so !node.getIsSyntaxHighlightSupported() evaluates to true when the value is null, triggering setIsSyntaxHighlightSupported(true), which marks the node dirty, re-triggers the transform, and loops forever. The fix uses strict equality (!== true, !== false) to correctly handle the null/undefined initial state without triggering redundant mutations.

What I checked:

• ✅ Logic correctness: Using strict inequality (!== true / !== false) correctly handles the three-state scenario (null | true | false) — only calls the setter when the value actually needs to change.
• ✅ Applied consistently across all 3 files (CodeHighlighterPrism.ts, CodeHighlighterShiki.ts, FacadeShiki.ts).
• ✅ www compat: No public API changes. Internal behavior fix only.

Issues:

• ❌ CLA check failing — must be signed before merge.
• ⚠️ Includes code.patch file — appears to be a stale patch file committed to the repo root. This should be removed before merge or moved to a documentation location.

Note: This PR overlaps with #8606 (same area, Shiki-only). If both land, there may be a merge conflict. #8606 takes a different approach (reordering the loadCodeLanguage call). The approaches are complementary — this PR's strict equality check is more defensive and covers both Prism and Shiki.