Handle popovers near edge of viewport about popover

[willshowell]

Glad to hear it's helping! Here's a demo of the issue (for my own reference later)

https://stackblitz.com/edit/sat-popover-issues-uwmunm?file=app%2Fapp.component.html

The cdk's positioning strategies have some planned upgrades, but who knows when they'll be available. Until then, I think this is something that can be fixed if we more intelligently configure fallback positions.

popover/src/lib/popover/popover-anchor.directive.ts

Lines 282 to 332 in dbb5211

	// Generate a position strategy with iterative fallback solutions
	// 1 2 3
	// ↖︎ => 4 5 6
	// 7 8 9
	return this._overlay.position()
	// Original Y position (1)
	.connectedTo(this._elementRef,
	{originX: originX, originY: originY},
	{overlayX: overlayX, overlayY: overlayY}
	)
	.withDirection(this._getDirection())
	// (2)
	.withFallbackPosition(
	{originX: 'center', originY: originY},
	{overlayX: 'center', overlayY: overlayY}
	)
	// (3)
	.withFallbackPosition(
	{originX: reverseHorizontal(originX), originY: originY},
	{overlayX: reverseHorizontal(overlayX), overlayY: overlayY},
	)
	// Center Y position (4)
	.withFallbackPosition(
	{originX: originX, originY: 'center'},
	{overlayX: overlayX, overlayY: 'center'}
	)
	// (5)
	.withFallbackPosition(
	{originX: 'center', originY: 'center'},
	{overlayX: 'center', overlayY: 'center'}
	)
	// (6)
	.withFallbackPosition(
	{originX: reverseHorizontal(originX), originY: 'center'},
	{overlayX: reverseHorizontal(overlayX), overlayY: 'center'},
	)
	// Reverse Y position (7)
	.withFallbackPosition(
	{originX: originX, originY: reverseVertical(originY)},
	{overlayX: overlayX, overlayY: reverseVertical(overlayY)}
	)
	// (8)
	.withFallbackPosition(
	{originX: 'center', originY: reverseVertical(originY)},
	{overlayX: 'center', overlayY: reverseVertical(overlayY)}
	)
	// (9)
	.withFallbackPosition(
	{originX: reverseHorizontal(originX), originY: reverseVertical(originY)},
	{overlayX: reverseHorizontal(overlayX), overlayY: reverseVertical(overlayY)}
	);

It works alright if you set the x and y positions somewhere other than center, but it doesn't do much if both x and y are 'center'.