webgl: Don't dirty canvas element while in immersive mode.

components/script/dom/navigator.rs

@@ -59,6 +59,10 @@ impl Navigator {
     pub fn new(window: &Window) -> DomRoot<Navigator> {
         reflect_dom_object(Box::new(Navigator::new_inherited()), window)
     }
+
+    pub fn xr(&self) -> Option<DomRoot<XRSystem>> {
+        self.xr.get()
+    }
 }
 
 impl NavigatorMethods for Navigator {

components/script/dom/webglrenderingcontext.rs

@@ -502,6 +502,12 @@ impl WebGLRenderingContext {
             return;
         }
 
+        // Dirtying the canvas is unnecessary if we're actively displaying immersive
+        // XR content right now.
+        if self.global().as_window().in_immersive_xr_session() {
+            return;
+        }
+
         self.canvas
             .upcast::<Node>()
             .dirty(NodeDamage::OtherNodeDamage);

components/script/dom/window.rs

@@ -2225,6 +2225,14 @@ impl Window {
     pub fn webrender_document(&self) -> DocumentId {
         self.webrender_document
     }
+
+    pub fn in_immersive_xr_session(&self) -> bool {
+        self.navigator
+            .get()
+            .as_ref()
+            .and_then(|nav| nav.xr())
+            .map_or(false, |xr| xr.pending_or_active_session())
+    }
 }
 
 impl Window {