Strip mentions from forwarded messages (#30884)

* strip mentions from forwarded messages

fixes element-hq/element-web#30883

* call attachMentions() for empty m.mentions in forwarded messages

As there is no EditorModel, attachMentions() currently does nothing

* fix lint and typecheck

* update test "should be navigable using arrow keys"

* update test "forwards pin drop event"

add empty mentions to expected content

* add doc to transformEvent() & elaborate on attachMentions()

* add test "strips mentions from forwarded messages"

* fix lint

* update source of `attachMentions()` import

---------

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

test/unit-tests/components/views/dialogs/ForwardDialog-test.tsx

@@ -153,8 +153,9 @@ describe("ForwardDialog", () => {
 
         await userEvent.keyboard("[Enter]");
         expect(mockClient.sendEvent).toHaveBeenCalledWith("A", "m.room.message", {
-            body: "Hello world!",
-            msgtype: "m.text",
+            "body": "Hello world!",
+            "msgtype": "m.text",
+            "m.mentions": {},
         });
     });
 
@@ -248,6 +249,37 @@ describe("ForwardDialog", () => {
         expect(secondButton.getAttribute("aria-disabled")).toBeFalsy();
     });
 
+    it("strips mentions from forwarded messages", async () => {
+        const messageWithMention = mkEvent({
+            type: "m.room.message",
+            room: sourceRoom,
+            user: "@bob:example.org",
+            content: {
+                "msgtype": "m.text",
+                "body": "Hi @alice:example.org",
+                "m.mentions": {
+                    user_ids: ["@alice:example.org"],
+                },
+            },
+            event: true,
+        });
+
+        const { container } = mountForwardDialog(messageWithMention);
+        const roomId = "a";
+
+        // Click the send button.
+        act(() => {
+            const sendButton = container.querySelector(".mx_ForwardList_sendButton");
+            fireEvent.click(sendButton!);
+        });
+
+        // Expected content should have mentions empty.
+        expect(mockClient.sendEvent).toHaveBeenCalledWith(roomId, messageWithMention.getType(), {
+            ...messageWithMention.getContent(),
+            "m.mentions": {},
+        });
+    });
+
     describe("Location events", () => {
         // 14.03.2022 16:15
         const now = 1647270879403;
@@ -357,11 +389,12 @@ describe("ForwardDialog", () => {
 
             sendToFirstRoom(container);
 
-            expect(mockClient.sendEvent).toHaveBeenCalledWith(
-                roomId,
-                pinDropLocationEvent.getType(),
-                pinDropLocationEvent.getContent(),
-            );
+            const expectedContent = {
+                ...pinDropLocationEvent.getContent(),
+                "m.mentions": {}, // Add mentions (explicitly set to empty)
+            };
+
+            expect(mockClient.sendEvent).toHaveBeenCalledWith(roomId, pinDropLocationEvent.getType(), expectedContent);
         });
     });