Scrolling in focused JavaScript embeds will still scroll the entire page

[muzikbike]

Describe the bug: If an embedded JS/HTML5 program has focus, and it uses the scroll wheel, the scroll wheel input will be passed to it, but the page it's embedded in will still scroll anyway.

To Reproduce: ./servo https://lazyslug.com/lifeview/plugin/viewer.html click on embed, use scroll wheel

Platform: see #41153

[jdm]

This may be related to how servo handles touch-action: none, or it may be related to the way we dispatch wheel events.

[jdm]

Minimized testcase:

<div style="width: 10vw; height: 40px; background-color: red;" id="e"></div>
<script>
const e = document.querySelector('#e');
e.onwheel = (ev) => ev.preventDefault();
</script>
<div style="width: 10vw; height: 200vh; background-color: green"></div>

In Firefox attempting to scroll over the red box will do nothing. In Servo the page will scroll.

[jdm]

I'm pretty sure the problem is this: https://github.com/servo/servo/blob/b7263e7fda4e10ae9d86a501c22d74f45138c60b/components/compositing/webview_renderer.rs#L393-L405

We dispatch the input event, but we immediately push a pending scroll event (in https://github.com/servo/servo/blob/b7263e7fda4e10ae9d86a501c22d74f45138c60b/components/compositing/webview_renderer.rs#L684-L696) without waiting to see if content called preventDefault.

https://github.com/servo/servo/blob/b7263e7fda4e10ae9d86a501c22d74f45138c60b/components/compositing/webview_renderer.rs#L1047 is the code that is called when content finishes processing an input event (such as a wheel event), so we could move the code that pushes the scrolling event into that method instead if the input result is not DefaultPrevented.