Scrollable areas - add dynamic shadows based on scroll position

从链接的可滚动页面部分 overflow-scroll demo 开始，检查 .pf-m-overflow-scroll 和 .pf-m-shadow-bottom 当前的行为。明确动态阴影应广泛应用，还是仅应用于页面部分，包括对性能的预期以及所需的滚动状态。在开始实现之前就范围和行为达成一致，即视为完成。

From @jamestalton, I wanted to create an issue to track a request I've heard from James a few times. It would be super cool if PF was able to add shadows to scrollable regions dynamically. A good example we could currently work with is scrollable page sections. You designate a page section as .pf-m-overflow-scroll, and by default, it looks like any other page section. However, if its content is taller than the section, it will automatically add a scrollbar and the content becomes scrollable. PF could detected that there is overflow and added a shadow dynamically. Currently users can add shadows themselves by adding .pf-m-shadow-bottom (one for a top shadow, too) to the preceding page section, but that class adds a static shadow. If you add it because the section below has the potential to scroll, but doesn't scroll because there is no overflow content, the shadow appears unnecessarily.

Some questions:

• Currently a user could wire this up themselves, but should PF provide this ability?

• Would this be enabled for all scrollable regions? Sounds like that has the potential to create a lot of work to implement/maintain.

• Is there any kind of performance cost?

• Are there other cases for dynamic shadows - for example, adding a shadow only once you've scrolled a little bit? What's the LEO for that and does it come with performance costs?

Anything else you'd like to add @jamestalton?

Jira Issue: PF-1775