dom: Stub out the IntersectionObserver interface (#33989)

This is the first step toward implementing the IntersectionObserver
interface. It adds stubs which are exposed when a preference is turned
on. This is enough to get some sites with `IntersectionObserver` to
start working.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/intersectionobserverentry.rs

@@ -0,0 +1,128 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use dom_struct::dom_struct;
+use js::rust::HandleObject;
+
+use super::bindings::codegen::Bindings::IntersectionObserverEntryBinding::{
+    IntersectionObserverEntryInit, IntersectionObserverEntryMethods,
+};
+use super::bindings::num::Finite;
+use crate::dom::bindings::reflector::{reflect_dom_object_with_proto, DomObject, Reflector};
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::domrectreadonly::DOMRectReadOnly;
+use crate::dom::element::Element;
+use crate::dom::window::Window;
+use crate::script_runtime::CanGc;
+
+/// An individual IntersectionObserver entry.
+///
+/// <https://w3c.github.io/IntersectionObserver/#intersection-observer-entry>
+#[dom_struct]
+pub struct IntersectionObserverEntry {
+    reflector_: Reflector,
+    target: Dom<Element>,
+}
+
+impl IntersectionObserverEntry {
+    pub fn new_inherited(init: &IntersectionObserverEntryInit) -> Self {
+        Self {
+            reflector_: Reflector::new(),
+            target: init.target.as_traced(),
+        }
+    }
+
+    fn new(
+        window: &Window,
+        proto: Option<HandleObject>,
+        init: &IntersectionObserverEntryInit,
+        can_gc: CanGc,
+    ) -> DomRoot<Self> {
+        let observer = Box::new(Self::new_inherited(init));
+        reflect_dom_object_with_proto(observer, window, proto, can_gc)
+    }
+}
+
+impl IntersectionObserverEntryMethods for IntersectionObserverEntry {
+    /// > The attribute must return a DOMHighResTimeStamp that corresponds to the time the
+    /// > intersection was recorded, relative to the time origin of the global object
+    /// > associated with the IntersectionObserver instance that generated the notification.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-time>
+    fn Time(&self) -> Finite<f64> {
+        Finite::new(0.).unwrap()
+    }
+
+    /// > For a same-origin-domain target, this will be the root intersection rectangle.
+    /// > Otherwise, this will be null. Note that if the target is in a different browsing
+    /// > context than the intersection root, this will be in a different coordinate system
+    /// > than boundingClientRect and intersectionRect.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-rootbounds>
+    fn GetRootBounds(&self) -> Option<DomRoot<DOMRectReadOnly>> {
+        None
+    }
+
+    /// > A DOMRectReadOnly obtained by getting the bounding box for target.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-boundingclientrect>
+    fn BoundingClientRect(&self) -> DomRoot<DOMRectReadOnly> {
+        DOMRectReadOnly::new(&self.global(), None, 0., 0., 0., 0., CanGc::note())
+    }
+
+    /// > boundingClientRect, intersected by each of target's ancestors' clip rects (up to
+    /// > but not including root), intersected with the root intersection rectangle. This
+    /// > value represents the portion of target that intersects with the root intersection
+    /// > rectangle.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-intersectionrect>
+    fn IntersectionRect(&self) -> DomRoot<DOMRectReadOnly> {
+        DOMRectReadOnly::new(&self.global(), None, 0., 0., 0., 0., CanGc::note())
+    }
+
+    /// > True if the target intersects with the root; false otherwise. This flag makes it
+    /// > possible to distinguish between an IntersectionObserverEntry signalling the
+    /// > transition from intersecting to not-intersecting; and an IntersectionObserverEntry
+    /// > signalling a transition from not-intersecting to intersecting with a zero-area
+    /// > intersection rect (as will happen with edge-adjacent intersections, or when the
+    /// > boundingClientRect has zero area).
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-isintersecting>
+    fn IsIntersecting(&self) -> bool {
+        false
+    }
+
+    /// > Contains the result of running the visibility algorithm on target.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-isvisible>
+    fn IsVisible(&self) -> bool {
+        false
+    }
+
+    /// > If the boundingClientRect has non-zero area, this will be the ratio of
+    /// > intersectionRect area to boundingClientRect area. Otherwise, this will be 1 if the
+    /// > isIntersecting is true, and 0 if not.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-intersectionratio>
+    fn IntersectionRatio(&self) -> Finite<f64> {
+        Finite::new(0.).unwrap()
+    }
+
+    /// > The Element whose intersection with the intersection root changed.
+    ///
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-target>
+    fn Target(&self) -> DomRoot<Element> {
+        self.target.as_rooted()
+    }
+
+    /// <https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-intersectionobserverentry>
+    fn Constructor(
+        window: &Window,
+        proto: Option<HandleObject>,
+        can_gc: CanGc,
+        init: &IntersectionObserverEntryInit,
+    ) -> DomRoot<IntersectionObserverEntry> {
+        Self::new(window, proto, init, can_gc)
+    }
+}