There is a `handle_event` virtual method on `Node` that triggers the
default event handler. Typically the "superclass" virtual method is
called and then the specific type runs its default event handler. This
isn't good for form controls, because typically the event handler should
prevent further event handlers from running. This badness can be
observed by pressing down in a text input, which will both move the
cursor as well as scroll the page. 🙃
This change makes it so that the default form control event handler
consistently calls the `Event::mark_as_handled` method and triggers the
"superclass" event handler *after* running its own. In addition, the
scrolling default event handler on `Node` now doesn't run if the `Event`
was already handled.
Finally, a little bit of dead event handling code is removed.
Testing: This change adds a Servo-specific test for this behavior.
Fixes: #40785
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
4c6dbd59cb