Warn on web if there are more than one viewport

crates/eframe/src/web/app_runner.rs

@@ -185,9 +185,14 @@ impl AppRunner {
             platform_output,
             textures_delta,
             shapes,
-            ..
+            viewports,
+            viewport_commands: _, // TODO(emilk): handle some of the commands, like setting the title and icon
         } = full_output;
 
+        if viewports.len() > 1 {
+            log::warn!("Multiple viewports not yet supported on the web");
+        }
+
         self.handle_platform_output(platform_output);
         self.textures_delta.append(textures_delta);
         let clipped_primitives = self.egui_ctx.tessellate(shapes);

crates/egui/src/viewport.rs

@@ -668,13 +668,19 @@ pub(crate) struct ViewportState {
 /// Describes a viewport, i.e. a native window.
 #[derive(Clone)]
 pub struct ViewportOutput {
-    pub builder: ViewportBuilder,
-
     /// Id of us and our parent.
     pub id_pair: ViewportIdPair,
 
+    pub builder: ViewportBuilder,
+
     /// The user-code that shows the GUI, used for deferred viewports.
     ///
     /// `None` for immediate viewports and the ROOT viewport.
     pub viewport_ui_cb: Option<Arc<ViewportUiCallback>>,
 }
+
+impl ViewportOutput {
+    pub fn id(&self) -> ViewportId {
+        self.id_pair.this
+    }
+}