Web: account for canvas being focused already (#3369)

CHANGELOG.md

@@ -11,6 +11,8 @@ Unreleased` header.
 
 # Unreleased
 
+- On Web, account for canvas being focused already before event loop starts.
+
 # 0.29.9
 
 - On X11, fix `NotSupported` error not propagated when creating event loop.

src/platform_impl/web/event_loop/window_target.rs

@@ -5,6 +5,8 @@ use std::iter;
 use std::marker::PhantomData;
 use std::rc::{Rc, Weak};
 
+use web_sys::Element;
+
 use super::runner::{EventWrapper, Execution};
 use super::{
     super::{monitor::MonitorHandle, KeyEventExtra},
@@ -122,6 +124,25 @@ impl<T> EventLoopWindowTarget<T> {
             }
         });
 
+        // It is possible that at this point the canvas has
+        // been focused before the callback can be called.
+        let focused = canvas
+            .document()
+            .active_element()
+            .filter(|element| {
+                let canvas: &Element = canvas.raw();
+                element == canvas
+            })
+            .is_some();
+
+        if focused {
+            canvas.has_focus.set(true);
+            self.runner.send_event(Event::WindowEvent {
+                window_id: RootWindowId(id),
+                event: WindowEvent::Focused(true),
+            })
+        }
+
         let runner = self.runner.clone();
         let modifiers = self.modifiers.clone();
         canvas.on_keyboard_press(