Grid Combobox Example: Improve popup trigger and scrolling

[mcking65]

Per review of combobox examples by @smhigley in #1255, the grid combobox has the following two undesireable behaviors:

1. It does not let you open the popup and see all options without first typing something.
2. The list scrolls every time you navigate to a new option, even if that option was already in view before the programmatic scroll.