Add pointer events and focus handling for apps run in a Shadow DOM (#5627)

* [x] I have followed the instructions in the PR template

This PR handles pointer events and focus which did following changes:
- `element_from_point` and focus is now acquired from root node object
by using `get_root_node` from document or a shadow root.
- `TextAgent` is appended individually in each shadow root.

These changes handles pointer events and focus well in a web app that
are running in a shadow dom, or else the hover pointer actions and
keyboard input events are not triggered in a shadow dom.
Helpful for building embeddable/multi-view web-apps.

crates/eframe/src/web/events.rs

@@ -1,5 +1,3 @@
-use web_sys::EventTarget;
-
 use crate::web::string_from_js_value;
 
 use super::{
@@ -10,6 +8,8 @@ use super::{
     DEBUG_RESIZE,
 };
 
+use web_sys::{Document, EventTarget, ShadowRoot};
+
 // TODO(emilk): there are more calls to `prevent_default` and `stop_propagation`
 // than what is probably needed.
 
@@ -570,10 +570,17 @@ fn install_pointerup(runner_ref: &WebRunner, target: &EventTarget) -> Result<(),
 /// Returns true if the cursor is above the canvas, or if we're dragging something.
 /// Pass in the position in browser viewport coordinates (usually event.clientX/Y).
 fn is_interested_in_pointer_event(runner: &AppRunner, pos: egui::Pos2) -> bool {
-    let document = web_sys::window().unwrap().document().unwrap();
-    let is_hovering_canvas = document
-        .element_from_point(pos.x, pos.y)
-        .is_some_and(|element| element.eq(runner.canvas()));
+    let root_node = runner.canvas().get_root_node();
+
+    let element_at_point = if let Some(document) = root_node.dyn_ref::<Document>() {
+        document.element_from_point(pos.x, pos.y)
+    } else if let Some(shadow) = root_node.dyn_ref::<ShadowRoot>() {
+        shadow.element_from_point(pos.x, pos.y)
+    } else {
+        None
+    };
+
+    let is_hovering_canvas = element_at_point.is_some_and(|element| element.eq(runner.canvas()));
     let is_pointer_down = runner
         .egui_ctx()
         .input(|i| i.pointer.any_down() || i.any_touches());