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/shared/embedder/lib.rs

@@ -22,7 +22,7 @@ use std::sync::Arc;
 
 use base::id::{PipelineId, ScrollTreeNodeId, WebViewId};
 use crossbeam_channel::Sender;
-use euclid::{Scale, Size2D};
+use euclid::{Point2D, Scale, Size2D};
 use http::{HeaderMap, Method, StatusCode};
 use ipc_channel::ipc::IpcSender;
 use log::warn;
@@ -770,10 +770,14 @@ pub struct CompositorHitTestResult {
     pub pipeline_id: PipelineId,
 
     /// The hit test point in the item's viewport.
-    pub point_in_viewport: euclid::default::Point2D<f32>,
+    pub point_in_viewport: Point2D<f32, CSSPixel>,
+
+    /// The hit test point relative to the root scroll node content origin / initial
+    /// containing block.
+    pub point_relative_to_initial_containing_block: Point2D<f32, CSSPixel>,
 
     /// The hit test point relative to the item itself.
-    pub point_relative_to_item: euclid::default::Point2D<f32>,
+    pub point_relative_to_item: Point2D<f32, CSSPixel>,
 
     /// The node address of the hit test result.
     pub node: UntrustedNodeAddress,