ELK runs in a worker: the flow layout loads elkjs's worker build as a Vite ?worker behind elk-api, so a phone's eight layout passes no longer block the page (Demo house at 390px with 4x CPU throttling, long tasks 15.0 to 0.8 s, longest 2.1 to 0.3 s); positions and routes unchanged

frontend/src/components/Flow/layout.check.ts

@@ -14,6 +14,20 @@ import assert from "node:assert/strict"
 // @ts-expect-error the runtime has it, the DOM types say it is not removable.
 globalThis.self = undefined
 
+// A page runs ELK in a worker Vite builds (`?worker`). Here the same build
+// runs in-thread, as the fake worker it exports outside a worker.
+// @ts-expect-error the worker build ships without types.
+const { Worker: InThread } = await import("elkjs/lib/elk-worker.min.js")
+// @ts-expect-error bun's global, which the DOM types do not know.
+Bun.plugin({
+  name: "elk in-thread",
+  setup: (build: { module: (id: string, load: () => object) => void }) =>
+    build.module("elkjs/lib/elk-worker.min.js?worker", () => ({
+      exports: { default: InThread },
+      loader: "object",
+    })),
+})
+
 const { layoutGraph, nodeHeight, portFraction, PORT_SPAN } = await import(
   "./layout"
 )

frontend/src/components/Flow/layout.ts

@@ -384,14 +384,20 @@ function wrapWideRanks(placed: Map<string, Point>, ids: string[]): Wrap[] {
  * The layout engine, fetched the first time a flow needs laying out.
  *
  * Its own chunk: ELK is a compiled Java library and weighs more than the rest
- * of the canvas put together, so nothing but this route ever loads it.
+ * of the canvas put together, so nothing but this route ever loads it. And
+ * its own thread: a phone lays the Demo house out eight times over, about
+ * half a second each, which on the page would freeze it for seconds.
  */
 let engine: Promise<{ layout: (graph: ElkNode) => Promise<ElkNode> }> | null =
   null
 
 function elk() {
-  engine ??= import("elkjs/lib/elk.bundled.js").then(
-    (module) => new module.default(),
+  engine ??= Promise.all([
+    import("elkjs/lib/elk-api.js"),
+    import("elkjs/lib/elk-worker.min.js?worker"),
+  ]).then(
+    ([api, worker]) =>
+      new api.default({ workerFactory: () => new worker.default() }),
   )
   return engine
 }