Start native worker only after CSS cache configuration #111

Problem

Enabling the compiled CSS cache in the Code OSS host exposes a startup-order race in webscene_engine. The constructor starts worker_ in its member initializer list, then configures the process CSS compilation-cache directory in the constructor body. The worker can begin loading and parsing the document before constructor setup is complete.

On the macOS 12 deployment-target build used by AppScene, the packaged VS Code OSS workbench remains in continuous visible rendering after startup:

• visible idle presentations: 119.010/sec
• visible idle CPU: 1.089%
• scene generation and publication continue at display refresh rate

The prior runtime and a build where the worker starts after cache setup both settle:

• visible idle presentations: 0/sec
• hidden idle presentations: 0/sec
• visible idle CPU: 0.042%

A macOS 26-target rebuild happened to mask the race, so deployment target and compiler timing affect reproduction.

Root cause

worker_ begins during member initialization. Process-wide CSS cache setup happens later in the constructor body, despite the class invariant that the worker should only observe fully configured state. Bypassing CSS event-cache replay still reproduces 119.048 presentations/sec, ruling out replay semantics.

Proposed fix

Default-construct the final std::jthread member. In the constructor body:

1. Configure the CSS compilation-cache directory.
2. Mark constructor setup complete.
3. Assign/start worker_.

Add a worker-entry invariant guard so future ordering regressions fail closed.

Regression coverage

• Repeatedly construct configured engines with a nonempty cache directory and execute a worker task.
• Assert the worker never observes partially configured engine state.
• Keep the packaged performance gates:
  • idle.visiblePresentationsPerSecond <= 5
  • idle.hiddenPresentationsPerSecond <= 2
• Validate with CMAKE_OSX_DEPLOYMENT_TARGET=12.0, where the race reproduces reliably.

Acceptance

• Native lifecycle regression passes.
• Existing native runtime, CSS parser/cache, WPT, and package gates pass.
• Code OSS reaches 0 idle visible/hidden presentations per second after warmup.
• No change to CSS cache contents or public ABI.