script: Pass more information to the MouseEvent constructor (#37672)

- Instead of eagerly computing `pageX` and `pageY`, collect the offset
  from the content's initial containing block in the compositor and pass
  that information through to `MouseEvent`. This prevents a layout flush
  that was happening when eagerly trying to fetch `Document` scroll
  offsets.
- Pass keyboard modifiers properly to `MouseEvent`.
- Now all this information is stored and passed as `Point2D` (typed) and
  `Modifiers` which greatly reduces the amount of arguments that need to
  be passed around.

Testing: It is difficult to test input events as they require WebDriver
which
isn't completely working yet. I have manually run Speedometer 2.1 and I
have
verified that this fixes the regression from #37601.
Fixes: #37601.

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

components/compositing/compositor.rs

@@ -336,11 +336,23 @@ impl ServoRenderer {
                     _ => return None,
                 }
 
+                let offset = details
+                    .scroll_tree
+                    .scroll_offset(pipeline_id.root_scroll_id())
+                    .unwrap_or_default();
+                let point_in_initial_containing_block =
+                    (item.point_in_viewport + offset).to_untyped();
+
                 let info = &details.hit_test_items[item.tag.0 as usize];
                 Some(CompositorHitTestResult {
                     pipeline_id,
-                    point_in_viewport: item.point_in_viewport.to_untyped(),
-                    point_relative_to_item: item.point_relative_to_item.to_untyped(),
+                    point_in_viewport: Point2D::from_untyped(item.point_in_viewport.to_untyped()),
+                    point_relative_to_initial_containing_block: Point2D::from_untyped(
+                        point_in_initial_containing_block,
+                    ),
+                    point_relative_to_item: Point2D::from_untyped(
+                        item.point_relative_to_item.to_untyped(),
+                    ),
                     node: UntrustedNodeAddress(info.node as *const c_void),
                     cursor: info.cursor,
                     scroll_tree_node: info.scroll_tree_node,