Auto merge of #27235 - asajeffrey:webxr-sessionavailable, r=jdm

Add a sessionavailable event that allows content to enter an XR session without a user gesture

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This allows servo to implement immersive web apps. https://immersive-web.github.io/webxr/#application-launch

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because I'm not sure we should have a WPT for something this non-standard.

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components/atoms/static_atoms.txt

@@ -115,6 +115,7 @@ selectend
 selectionchange
 selectstart
 serif
+sessionavailable
 signalingstatechange
 squeeze
 squeezeend

components/config/prefs.rs

@@ -280,7 +280,8 @@ mod gen {
                     },
                     layers: {
                         enabled: bool,
-                    }
+                    },
+                    sessionavailable: bool,
                 },
                 worklet: {
                     blockingsleep: {

components/script/dom/document.rs

@@ -17,6 +17,7 @@ use crate::dom::bindings::codegen::Bindings::EventBinding::EventBinding::EventMe
 use crate::dom::bindings::codegen::Bindings::HTMLIFrameElementBinding::HTMLIFrameElementBinding::HTMLIFrameElementMethods;
 use crate::dom::bindings::codegen::Bindings::HTMLInputElementBinding::HTMLInputElementMethods;
 use crate::dom::bindings::codegen::Bindings::HTMLTextAreaElementBinding::HTMLTextAreaElementMethods;
+use crate::dom::bindings::codegen::Bindings::NavigatorBinding::NavigatorBinding::NavigatorMethods;
 use crate::dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
 use crate::dom::bindings::codegen::Bindings::NodeFilterBinding::NodeFilter;
 use crate::dom::bindings::codegen::Bindings::PerformanceBinding::PerformanceMethods;
@@ -2267,6 +2268,17 @@ impl Document {
         // Step 11.
         // TODO: ready for post-load tasks.
 
+        // The dom.webxr.sessionavailable pref allows webxr
+        // content to immediately begin a session without waiting for a user gesture.
+        // TODO: should this only happen on the first document loaded?
+        // https://immersive-web.github.io/webxr/#user-intention
+        // https://github.com/immersive-web/navigation/issues/10
+        if pref!(dom.webxr.sessionavailable) {
+            if self.window.is_top_level() {
+                self.window.Navigator().Xr().dispatch_sessionavailable();
+            }
+        }
+
         // Step 12: completely loaded.
         // https://html.spec.whatwg.org/multipage/#completely-loaded
         // TODO: fully implement "completely loaded".

components/script/dom/xrsystem.rs

@@ -7,6 +7,7 @@ use crate::dom::bindings::codegen::Bindings::XRSystemBinding::XRSessionInit;
 use crate::dom::bindings::codegen::Bindings::XRSystemBinding::{XRSessionMode, XRSystemMethods};
 use crate::dom::bindings::conversions::{ConversionResult, FromJSValConvertible};
 use crate::dom::bindings::error::Error;
+use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::refcounted::{Trusted, TrustedPromise};
 use crate::dom::bindings::reflector::{reflect_dom_object, DomObject};
 use crate::dom::bindings::root::{Dom, DomRoot, MutNullableDom};
@@ -289,4 +290,26 @@ impl XRSystem {
         // This must be called _after_ the promise is resolved
         session.setup_initial_inputs();
     }
+
+    // https://github.com/immersive-web/navigation/issues/10
+    pub fn dispatch_sessionavailable(&self) {
+        let xr = Trusted::new(self);
+        let global = self.global();
+        let window = global.as_window();
+        window
+            .task_manager()
+            .dom_manipulation_task_source()
+            .queue(
+                task!(fire_sessionavailable_event: move || {
+                    // The sessionavailable event indicates user intent to enter an XR session
+                    let xr = xr.root();
+                    let interacting = ScriptThread::is_user_interacting();
+                    ScriptThread::set_user_interacting(true);
+                    xr.upcast::<EventTarget>().fire_bubbling_event(atom!("sessionavailable"));
+                    ScriptThread::set_user_interacting(interacting);
+                }),
+                window.upcast(),
+            )
+            .unwrap();
+    }
 }