Make keyboard handling in and out of autocomplete completions consistent (#11344)

* Make keyboard handling in and out of autocomplete completions consistent

* Add test

test/components/views/rooms/BasicMessageComposer-test.tsx

@@ -82,6 +82,18 @@ describe("BasicMessageComposer", () => {
             expect(transformedText).toBe(after + " ");
         }
     });
+
+    it("should not mangle shift-enter when the autocomplete is open", async () => {
+        const model = new EditorModel([], pc, renderer);
+        render(<BasicMessageComposer model={model} room={room} />);
+
+        const input = screen.getByRole("textbox");
+
+        await userEvent.type(input, "/plain foobar");
+        await userEvent.type(input, "{Shift>}{Enter}{/Shift}");
+        const transformedText = model.parts.map((part) => part.text).join("");
+        expect(transformedText).toBe("/plain foobar\n");
+    });
 });
 
 function generateMockDataTransferForString(string: string): DataTransfer {