Fix playground diff highlights disappearing when navigating between output files

[Copilot]

Diff highlights in the playground output viewer were only visible for the file open at the moment compilation finished. Navigating to any other changed file showed the new content but no highlights, until the next compile.

Root cause

In OutputEditor, the decoration useEffect only depended on [changedLineNumbers, editorInstance]. On file navigation, setFilename is sync but loadOutputFile → setContent is async, producing two renders:

1. changedLineNumbers ref changes → decorations applied to the still‑stale model content.
2. value updates with the file's real content. model.setValue(value) replaces all content (discarding decorations), but the decoration effect doesn't re‑run because changedLineNumbers ref is unchanged → highlights are lost.

The originally‑open file works only because its content load and the change‑set update land in the same batched render, so decorations are applied after the final setValue.

Changes

• packages/playground/src/react/typespec-editor.tsx — Add filename and value to the decoration effect's dep array so decorations are re‑applied after the model is swapped or its content is replaced. Effects run in declaration order, and the existing setValue effect is declared first, so highlights are re‑applied last and survive. The editor is readOnly, so value only changes when the parent supplies new content — no churn on user input.
• Changelog — fix entry for @typespec/playground.

useEffect(() => {
  if (!editorInstance || !decorationCollectionRef.current) return;
  // ...apply or clear decorations...
}, [changedLineNumbers, editorInstance, filename, value]);
//                                       ^^^^^^^^^^^^^^^^ added

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/playground@10480

commit: ca44376

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[timotheeguerin]

Suggested change

	changeKind: fix
	changeKind: internal

this was added in the same release so no need to document the change

[timotheeguerin]

looking at the content of the effect not sure this is the right fix, worried this is just going to trigger at the wrong time causing more inconsitency on how things are recomputed. Neither filename or value are used in the effect which would be the reason for passing them in the trigger list