Theia: [editor] dismiss context menu on scroll

[AlexTugarev]

I was expecting either system behavior (no scrolling while context menu is visible) or the same as in monaco playground:

but currently the context menu just stays on top at the fixed position if you try scrolling:

the UX issue is that you're moving away from the original selection, which is context of that menu.

[kaiyue0329]

@akosyakov I was wondering if there is a way to prevent scrolling in an editor widget (or more specifically monaco editor)?

[akosyakov]

I think we should align with the playground. Probably listen to on scroll event and close the context menu.