SelectionList: Option should scroll into view when focused with the keyboard

[benjamincharity]

Currently, if the user uses the arrow keys to move down the list of options, they can move their selection out of view:

The focused option should scroll into view as the user moves down the list.

See

terminus-ui/terminus-ui/autocomplete/src/autocomplete-panel/autocomplete-trigger.directive.ts

Lines 759 to 772 in b36c77a

	private scrollToOption(): void {
	const index: number = this.autocompletePanel.keyManager.activeItemIndex || 0;
	const labelCount: number = countGroupLabelsBeforeOption(index, this.autocompletePanel.options, this.autocompletePanel.optionGroups);
	const newScrollPosition = getOptionScrollPosition(
	index + labelCount,
	// FIXME: can this height be dynamic like the TsSelectComponent? https://github.com/GetTerminus/terminus-ui/issues/1153
	AUTOCOMPLETE_OPTION_HEIGHT,
	this.autocompletePanel.getScrollTop(),
	AUTOCOMPLETE_PANEL_HEIGHT,
	);
	this.autocompletePanel.setScrollTop(newScrollPosition);
	}

---

• Add functionality to scroll focused option into view
• Add test