GestureEvent support in web-sys: wasm-bindgen/wasm-bindgen#4589
Alternatively, js_sys::Reflect could be used to dynamically extract the fields.

I implemented GestureEvent support utilizing js_sys::Reflect since it is a non-standard API. With that, both of eframe's backends now support rotation input in this PR.

Preview available at https://egui-pr-preview.github.io/pr/7453-rotationgesture
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

The Multi Touch demo could respond to smooth_scroll_delta, zoom_delta, and rotation_delta. However, all three of those will yield input at the same time as multi_touch, which would make the existing processing redundant. Also, the current touches label might be confusing if it is left as reporting zero touches (since no multi_touch exists) while the state is changing.

We could rename Multi Touch to something else, like Trackpad and Multitouch and then adapt its UI accordingly 🤔

Keeping it called Multi Touch isn't entirely inaccurate. The relevant events are still produced by the same physical actions, just on a trackpad instead of a touch screen. The current touches label could be swapped out to say Input source: none, Input source: 2-finger touch, or Input source: cursor depending on the incoming events.

I have also been working on some changes in winit to support multitouch trackpad gestures on Wayland, which will generate PinchGesture, RotateGesture, and PanGesture. Notably, PanGesture is the same physical action as MouseWheel. Which one is sent depends on if scrolling is detected first or if pinch/rotation is detected first . Therefore, a PanGesture from winit should probably be mapped to scrolling input in egui.