resolve undefined this in onClick handler (#32576)

* resolve undefined this in onClick handler

* add regression test for bound DisambiguatedProfileViewModel onClick

* Update docs

* add regression for sender profile click mention insertion

* Eslint Fix

docs/MVVM.md

@@ -127,6 +127,20 @@ export class FooViewModel extends BaseViewModel<FooViewSnapshot, Props> implemen
 }
 ```
 
+#### Binding of View Model Actions:
+
+All view model actions must be defined as arrow functions to ensure they are bound to the class instance.
+
+Using standard class methods can result in `this` being undefined when the function is passed as a callback (e.g. to a React event handler), which may cause runtime errors.
+
+Correct pattern:
+
+```ts
+public doSomething = (): void => {
+    ...
+};
+```
+
 ### `useViewModel` hook
 
 Your view must call this hook with the view-model as argument. Think of this as your view subscribing to the view model.<br>

playwright/e2e/timeline/timeline.spec.ts

@@ -1008,6 +1008,34 @@ test.describe("Timeline", () => {
             await expect(page.getByRole("button", { name: "Show video" })).toBeVisible();
             await expect(page.locator("video")).not.toBeVisible();
         });
+
+        test("should insert a mention when clicking sender profile in timeline", async ({
+            page,
+            app,
+            homeserver,
+            room,
+        }) => {
+            const senderDisplayName = "SenderBot";
+            const messageFromSender = "message from sender";
+
+            const bot = new Bot(page, homeserver, {
+                displayName: senderDisplayName,
+                autoAcceptInvites: false,
+            });
+            await bot.prepareClient();
+            await app.client.inviteUser(room.roomId, bot.credentials.userId);
+            await bot.joinRoom(room.roomId);
+            await bot.sendMessage(room.roomId, messageFromSender);
+
+            await app.viewRoomById(room.roomId);
+
+            const senderMessageTile = getEventTilesWithBodies(page).filter({ hasText: messageFromSender }).first();
+            await expect(senderMessageTile).toBeVisible();
+
+            await senderMessageTile.locator(".mx_DisambiguatedProfile").click();
+
+            await expect(app.getComposerField().getByText(senderDisplayName)).toBeVisible();
+        });
     });
 
     test.describe("message sending", { tag: ["@no-firefox", "@no-webkit"] }, () => {

src/viewmodels/profile/DisambiguatedProfileViewModel.ts

@@ -141,7 +141,7 @@ export class DisambiguatedProfileViewModel
         this.snapshot.set(DisambiguatedProfileViewModel.computeSnapshot(this.props));
     }
 
-    public onClick(evt: MouseEvent<HTMLDivElement>): void {
+    public onClick = (evt: MouseEvent<HTMLDivElement>): void => {
         this.props.onClick?.(evt);
-    }
+    };
 }

test/viewmodels/profile/DisambiguatedProfileViewModel-test.tsx

@@ -75,13 +75,27 @@ describe("DisambiguatedProfileViewModel", () => {
         const subscriber = jest.fn();
 
         vm.subscribe(subscriber);
-        onClick({} as never);
+        vm.onClick?.({} as never);
 
         expect(onClick).toHaveBeenCalledTimes(1);
         expect(subscriber).not.toHaveBeenCalled();
         expect(vm.getSnapshot()).toBe(prevSnapshot);
     });
 
+    it("should keep onClick bound when extracted as a callback", () => {
+        const onClick = jest.fn();
+        const vm = new DisambiguatedProfileViewModel({
+            member,
+            fallbackName: "Fallback",
+            onClick,
+        });
+
+        const clickHandler = vm.onClick;
+
+        expect(() => clickHandler?.({} as never)).not.toThrow();
+        expect(onClick).toHaveBeenCalledTimes(1);
+    });
+
     it("should emit snapshot update when fallbackName changes", () => {
         const vm = new DisambiguatedProfileViewModel({
             member: null,