Do not fire anywidget change listeners at mount

frontend/src/plugins/impl/anywidget/__tests__/host.test.ts

@@ -277,25 +277,22 @@ describe("WidgetBinding receives a host in render props", () => {
   });
 });
 
-describe("host-mounted views hydrate like display-mounted ones", () => {
-  it("replays current state to a child's render listeners", async () => {
-    const childId = asModelId("host-hydration-child");
+describe("host-mounted views", () => {
+  it("render with the child's current model state", async () => {
+    const childId = asModelId("host-child-state");
     const parentController = new AbortController();
     const getModuleSpy = vi.spyOn(WIDGET_DEF_REGISTRY, "getModule");
     try {
       const childWidget = {
         render: vi.fn(({ model, el }) => {
-          el.textContent = "count is 5";
-          model.on("change:count", () => {
-            el.textContent = `count is ${model.get("count")}`;
-          });
+          el.textContent = `count is ${model.get("count")}`;
         }),
       };
       const childModel = new Model<ModelState>({ count: 8 }, createMockComm());
       WIDGET_REGISTRY.setModel(childId, childModel);
       WIDGET_REGISTRY.setSpec(childId, {
-        url: "./@file/10-host-hydration-child.js",
-        hash: "hash-host-hydration-child",
+        url: "./@file/10-host-child-state.js",
+        hash: "hash-host-child-state",
       });
       getModuleSpy.mockResolvedValue({ default: childWidget });
 

frontend/src/plugins/impl/anywidget/__tests__/widget-binding.test.ts

@@ -337,13 +337,12 @@ describe("WidgetBinding", () => {
       { el: document.createElement("div") },
       { signal: controller.signal },
     );
-    // Called once by the hydration replay at mount, once by the set.
     model.set("count", 1);
-    expect(onCount).toHaveBeenCalledTimes(2);
+    expect(onCount).toHaveBeenCalledTimes(1);
 
     controller.abort();
     model.set("count", 2);
-    expect(onCount).toHaveBeenCalledTimes(2);
+    expect(onCount).toHaveBeenCalledTimes(1);
   });
 
   it("should auto-clear initialize listeners on destroy", async () => {
@@ -366,92 +365,54 @@ describe("WidgetBinding", () => {
   });
 });
 
-describe("WidgetBinding hydration replay", () => {
-  it("replays current state to render listeners exactly once", async () => {
+describe("WidgetBinding render listeners", () => {
+  it("fire only for changes after mount, never for pre-existing state", async () => {
     const model = new Model<ModelState>({ count: 8 }, createMockComm());
-    const el = document.createElement("div");
-    const widgetDef = {
-      render: vi.fn(({ model, el }) => {
-        // A widget view that starts with a local default and relies on
-        // change events for hydration.
-        el.textContent = "count is 5";
-        model.on("change:count", () => {
-          el.textContent = `count is ${model.get("count")}`;
-        });
-      }),
-    };
-    const binding = await createBinding(widgetDef, model);
-
-    const controller = new AbortController();
-    await binding.createView({ el }, { signal: controller.signal });
-    expect(el.textContent).toBe("count is 8");
-  });
-
-  it("does not re-fire listeners of already-mounted views", async () => {
-    // Mounting view B must not double-paint view A: the replay is
-    // scoped to the listeners the new render attached.
-    const model = new Model<ModelState>({ count: 0 }, createMockComm());
-    const listeners: Array<ReturnType<typeof vi.fn>> = [];
+    const onCount = vi.fn();
+    const onAnyChange = vi.fn();
     const widgetDef = {
       render: vi.fn(({ model }) => {
-        const listener = vi.fn();
-        listeners.push(listener);
-        model.on("change:count", listener);
+        model.on("change:count", onCount);
+        model.on("change", onAnyChange);
       }),
     };
     const binding = await createBinding(widgetDef, model);
     const controller = new AbortController();
-
     await binding.createView(
       { el: document.createElement("div") },
       { signal: controller.signal },
     );
-    expect(listeners[0]).toHaveBeenCalledTimes(1);
+    expect(onCount).not.toHaveBeenCalled();
+    expect(onAnyChange).not.toHaveBeenCalled();
 
-    await binding.createView(
-      { el: document.createElement("div") },
-      { signal: controller.signal },
-    );
-    // View B's listener hydrated once; view A's was left alone.
-    expect(listeners[1]).toHaveBeenCalledTimes(1);
-    expect(listeners[0]).toHaveBeenCalledTimes(1);
+    model.set("count", 9);
+    expect(onCount).toHaveBeenCalledTimes(1);
+    expect(onCount).toHaveBeenCalledWith(9);
   });
 
-  it("replays the any-change event to its listeners", async () => {
-    const model = new Model<ModelState>({ count: 1 }, createMockComm());
-    const onAnyChange = vi.fn();
+  it("mounting a second view does not fire the first view's listeners", async () => {
+    const model = new Model<ModelState>({ count: 0 }, createMockComm());
+    const listeners: Array<ReturnType<typeof vi.fn>> = [];
     const widgetDef = {
       render: vi.fn(({ model }) => {
-        model.on("change", onAnyChange);
+        const listener = vi.fn();
+        listeners.push(listener);
+        model.on("change:count", listener);
       }),
     };
     const binding = await createBinding(widgetDef, model);
     const controller = new AbortController();
+
     await binding.createView(
       { el: document.createElement("div") },
       { signal: controller.signal },
     );
-    expect(onAnyChange).toHaveBeenCalledTimes(1);
-  });
-
-  it("does not replay initialize listeners", async () => {
-    // The guarantee is per-view: initialize listeners existed before
-    // any view, and replaying at them would fire once per mount.
-    const model = new Model<ModelState>({ count: 1 }, createMockComm());
-    const initListener = vi.fn();
-    const widgetDef = {
-      initialize: vi.fn(({ model }) => {
-        model.on("change:count", initListener);
-      }),
-      render: vi.fn(),
-    };
-    const binding = await createBinding(widgetDef, model);
-    const controller = new AbortController();
     await binding.createView(
       { el: document.createElement("div") },
       { signal: controller.signal },
     );
-    expect(initListener).not.toHaveBeenCalled();
+    expect(listeners[0]).not.toHaveBeenCalled();
+    expect(listeners[1]).not.toHaveBeenCalled();
   });
 
   it("clears the element before rendering into it", async () => {

frontend/src/plugins/impl/anywidget/model-proxy.ts

@@ -5,15 +5,6 @@ import type { ModelState } from "./types";
 
 type ModelEventCallback = Parameters<AnyModel<ModelState>["on"]>[1];
 
-/**
- * A listener registered through a model proxy (see the hydration
- * replay in `WidgetBinding.createView`).
- */
-export interface ProxyRegistration {
-  event: string;
-  callback: ModelEventCallback;
-}
-
 /**
  * Wrap a model so every `on()` call from inside `initialize` or `render`
  * is auto-tied to a lifetime `AbortSignal`. When the signal aborts,
@@ -27,14 +18,11 @@ export interface ProxyRegistration {
  * The proxy is purely a host-side ergonomic helper. The widget author
  * still writes `model.on("change:foo", handler)` exactly as before; the
  * cleanup signal is supplied transparently.
- *
- * `onRegister`, when given, observes each `on()` registration.
  */
 // oxlint-disable-next-line marimo/prefer-object-params -- concise internal helper used at protocol call sites
 export function modelProxy<T extends ModelState>(
   model: AnyModel<T>,
   signal: AbortSignal,
-  onRegister?: (registration: ProxyRegistration) => void,
 ): AnyModel<T> {
   return {
     get(key) {
@@ -55,7 +43,6 @@ export function modelProxy<T extends ModelState>(
       signal.addEventListener("abort", () => model.off(name, callback), {
         once: true,
       });
-      onRegister?.({ event: name, callback });
     },
     off(name?: string | null, callback?: ModelEventCallback | null): void {
       model.off(name ?? null, callback ?? null);

frontend/src/plugins/impl/anywidget/widget-binding.ts

@@ -12,7 +12,7 @@ import { isTrustedVirtualFileUrl } from "@/plugins/core/trusted-url";
 import { Logger } from "@/utils/Logger";
 import type { Host } from "./host";
 import type { Model } from "./model";
-import { modelProxy, type ProxyRegistration } from "./model-proxy";
+import { modelProxy } from "./model-proxy";
 import type { ModelState } from "./types";
 
 export const experimental: Experimental = {
@@ -266,9 +266,8 @@ export class WidgetBinding<T extends ModelState = ModelState> {
    * when the caller's `signal` fires or the binding is destroyed, and
    * listeners registered inside `render` auto-clear on abort.
    *
-   * Hydration guarantee: listeners attached during `render` observe
-   * current model state exactly once, after `render` settles. Scoped
-   * to this view's listeners; re-firing at other views double-paints.
+   * `render` reads current state via `model.get`; change listeners
+   * observe only subsequent changes, matching Jupyter semantics.
    */
   async createView(
     target: { el: HTMLElement },
@@ -305,11 +304,8 @@ export class WidgetBinding<T extends ModelState = ModelState> {
     // Each view gets a host scoped to its own signal so child views tear
     // down with this view.
     const host = this.#createHost(renderSignal);
-    const registrations: ProxyRegistration[] = [];
     const renderCleanup = await widget.render({
-      model: modelProxy(this.#model, renderSignal, (registration) =>
-        registrations.push(registration),
-      ),
+      model: modelProxy(this.#model, renderSignal),
       el: target.el,
       experimental,
       signal: renderSignal,
@@ -333,7 +329,6 @@ export class WidgetBinding<T extends ModelState = ModelState> {
         once: true,
       });
     }
-    this.#replayState(registrations, renderSignal);
   }
 
   #trackCleanup(cleanup: Cleanup, reason: string): Promise<void> {
@@ -343,31 +338,6 @@ export class WidgetBinding<T extends ModelState = ModelState> {
     return task;
   }
 
-  /**
-   * The hydration guarantee documented on `createView`.
-   */
-  #replayState(
-    registrations: readonly ProxyRegistration[],
-    renderSignal: AbortSignal,
-  ): void {
-    const changePrefix = "change:";
-    for (const { event, callback } of registrations) {
-      if (renderSignal.aborted) {
-        return;
-      }
-      try {
-        if (event.startsWith(changePrefix)) {
-          const key = event.slice(changePrefix.length);
-          callback(this.#model.get(key));
-        } else if (event === "change") {
-          callback();
-        }
-      } catch (error) {
-        Logger.error("[WidgetBinding] Error replaying state", error);
-      }
-    }
-  }
-
   /**
    * Destroy this generation, running initialize/render cleanups and
    * clearing listeners registered through its model proxies.