Fix pinch-to-zoom on web by using the "artificial" modifier keys (#4619)

* Introduced in https://github.com/emilk/egui/pull/4524
* Closes https://github.com/emilk/egui/issues/4615

crates/eframe/src/web/events.rs

@@ -500,7 +500,10 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
         };
         // delta sign is flipped to match native (winit) convention.
         let delta = -egui::vec2(event.delta_x() as f32, event.delta_y() as f32);
-        let modifiers = runner.input.raw.modifiers;
+
+        // NOTE: pinch-to-zoom on a trackpad will set the `ctrl` modifier on the event,
+        // even though the user is not holding down ctrl!
+        let modifiers = modifiers_from_wheel_event(&event);
 
         runner.input.raw.events.push(egui::Event::MouseWheel {
             unit,

crates/eframe/src/web/input.rs

@@ -156,3 +156,19 @@ pub fn modifiers_from_mouse_event(event: &web_sys::MouseEvent) -> egui::Modifier
         command: event.ctrl_key() || event.meta_key(),
     }
 }
+
+pub fn modifiers_from_wheel_event(event: &web_sys::WheelEvent) -> egui::Modifiers {
+    egui::Modifiers {
+        alt: event.alt_key(),
+        ctrl: event.ctrl_key(),
+        shift: event.shift_key(),
+
+        // Ideally we should know if we are running or mac or not,
+        // but this works good enough for now.
+        mac_cmd: event.meta_key(),
+
+        // Ideally we should know if we are running or mac or not,
+        // but this works good enough for now.
+        command: event.ctrl_key() || event.meta_key(),
+    }
+}