Indicate that select menus are scrollable

[brandonreid]

In this issue, a user was not aware of additional options or the ability to scroll. I propose 2 options here.

The simple/quick option

We adopt a design principle for this component around applying max heights that include a peaking option. So perhaps the component is smart enough to know the height of options and self assign a max height that is something like true max minus half of one option. Or we simply state that this should be the desired outcome and the implementation assigns the max height that allows for a peeking option.

The over-engineered option

When options go higher than the max height of the container we add a scroll indicator:

Here you see a gradient overlay and two arrows indicating there is more to see. We'd need to clear this indicator any time a scroll event takes place. There's likely some subtlety to play with here around mouse over events, maybe the indicator slowly fades away.