script/compositor: Send mouseleave events when cursor moves between <iframe>s (#38539)

Properly send `mouseleave` events when the cursor moves between
`<iframe>`s. This allows a better handling of cursor changes and status
text updates. Specifically, we do not need to continuously update the
cursor and the value can be cached in the `Document`. In addition,
status updates can now be sent properly when moving focus between
`<iframe>`s.

Note that style updates for `:hover` values are still broken, but less
so than before. Now the hover state on the `Node` is updated, but for
some
reason the restyle isn't taking place properly. This maintains the
status quo as far as behavior goes when hover moves between `<iframe>`s.

This change also adds a helper data structure to `Document` which will
eventually be responsible for event handling.

Testing: Cursor and status change are currently very hard to test as
the API test harness makes this difficult at the moment.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/script/dom/document_event_handler.rs

@@ -0,0 +1,29 @@
+/* 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 std::cell::Cell;
+
+use embedder_traits::Cursor;
+use euclid::Point2D;
+use style_traits::CSSPixel;
+
+use crate::dom::bindings::root::MutNullableDom;
+use crate::dom::types::Element;
+
+/// The [`DocumentEventHandler`] is a structure responsible for handling events for
+/// the [`crate::Document`] and storing data related to event handling. It exists to
+/// decrease the size of the [`crate::Document`] structure.
+#[derive(Default, JSTraceable, MallocSizeOf)]
+#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
+pub(crate) struct DocumentEventHandler {
+    /// The element that is currently hovered by the cursor.
+    pub(crate) current_hover_target: MutNullableDom<Element>,
+    /// The most recent mouse movement point, used for processing `mouseleave` events.
+    #[no_trace]
+    pub(crate) most_recent_mousemove_point: Point2D<f32, CSSPixel>,
+    /// The currently set [`Cursor`] or `None` if the `Document` isn't being hovered
+    /// by the cursor.
+    #[no_trace]
+    pub(crate) current_cursor: Cell<Option<Cursor>>,
+}