Main-thread bridges for the browser editor, which is built as a whole-app worker. More...

Detailed Description

Main-thread bridges for the browser editor, which is built as a whole-app worker.

In that build main() runs on a pthread (Emscripten PROXY_TO_PTHREAD) and the single #canvas is transferred to it as an OffscreenCanvas. EM_JS bodies run in the JS context of the calling thread, so a window, document, or navigator reference in an EM_JS body would evaluate in a DedicatedWorkerGlobalScope where those globals do not exist. This header owns every browser-page seam the editor uses instead.

Two mechanisms, chosen per seam by how often it is read:

Shared-memory mirror. Values the app thread reads every frame (CSS viewport size, device pixel ratio, the frame-request flag, the wheel zoom modifier shadow) are published by main-thread DOM listeners writing directly into linear memory, which is shared across all pthreads. Reads on the app thread are ordinary loads. A synchronous proxied call per read would cost a round trip on every frame, which is exactly the budget this phase is trying to measure.
Async proxied JS. Values the page must observe but the app thread only writes (the window.__donner* probe surface) go through MAIN_THREAD_ASYNC_EM_ASM, which posts to the main thread without blocking the app thread.

Nothing here is compiled unless DONNER_EDITOR_WHOLE_APP_WORKER is defined, which the browser editor build always defines.