Not able to open context menu in drag and drop demo by right-click

**Describe the bug**
I am using code inspired by the drag and drop demo, and since updating from egui `0.19.0` to the most recent version, the context menu of the items seems not to work as expected anymore. I re-created this on [egui.rs](https://www.egui.rs/) (accessed 2022-12-17). When right-clicking on `Item B` (see video below, mouse pointer unfortunately not visible in the recording), a context menu with a `Remove` button as per https://github.com/emilk/egui/blob/6ae4bc486b2608212f5fc8453f5aa7e5106853f8/crates/egui_demo_lib/src/demo/drag_and_drop.rs#L135 should appear, but instead the `New Item` option appears.

[Screencast from 2022-12-17 10-56-00.webm](https://user-images.githubusercontent.com/54188922/208236399-369d7ba4-987c-46f5-b620-3072918b95f4.webm)

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://www.egui.rs/
2. Open the drag and drop demo
3. Right click on any of the items

Start at crates/egui_demo_lib/src/demo/drag_and_drop.rs around the referenced line, then reproduce the behavior in the drag and drop demo at egui.rs by right-clicking an item. Done means an item's context menu shows Remove instead of New Item, while the New Item option remains available in its intended context.

Written by the indexing model from the issue text.