This changes makes a variety of changes to ensure that the cursor is set
back to the default cursor when it leaves the `WebView`:
1. Display list updates can come after a mouse leaves the `WebView`, so
when refreshing the cursor after the update, base the updated cursor
on the last hovered location in the `DocumentEventHandler`, rather
than the compositor. This allows us to catch when the last hovered
position is `None` (ie the cursor has left the `WebView`).
2. When handling `MouseLeftViewport` events for the cursor leaving the
entire WebView, properly set the
MouseLeftViewport::focus_moving_to_another_iframe` on the input event
passed to the script thread.
3. When moving out of the `WebView` entirely, explicitly ask the
embedder to set the cursor back to the default.
Testing: This change adds a unit test verifying this behavior.
Fixes: #38710.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
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