Specify how scrolling works for form controls / widgets

[zcorpan]

From #4082 (comment)

Is <input type=text> a scroll container? What about other widgets?

Tentative test at web-platform-tests/wpt#20172

The test sets different 'overflow' values on input (with different type), and then calls scrollTo and listens for scroll events. Since it's not well defined how the widgets are supposed to be implemented, it's unclear if scrollTo should work at all and if a scroll event should be fired. But this happens in browsers for many input types today, but it's not interoperable.

Related PR: #4840

cc @whatwg/css