perf(tooltip): improve placement logic

[nickofthyme]

Summary

Fixes performance issue notice with synced tooltips.

Details

The popperSettings value here...

elastic-charts/packages/charts/src/components/tooltip/tooltip.tsx

Lines 150 to 167 in 48cade4

	const popperSettings = useMemo((): TooltipPortalSettings | undefined => {
	if (!settings || typeof settings === 'string') {
	return;
	}
	const { placement, fallbackPlacements, boundary, ...rest } = settings;
	return {
	...rest,
	placement: placement ?? (rotation === 0 || rotation === 180 ? Placement.Right : Placement.Top),
	fallbackPlacements:
	fallbackPlacements ??
	(rotation === 0 || rotation === 180
	? [Placement.Right, Placement.Left, Placement.Top, Placement.Bottom]
	: [Placement.Top, Placement.Bottom, Placement.Right, Placement.Left]),
	boundary: boundary === 'chart' ? chartRef.current ?? undefined : boundary,
	};
	}, [settings, chartRef, rotation]);

...was triggering an update for every new render even though the settings values had not changed.

This was caused by the getTooltipSettings helper method here...

elastic-charts/packages/charts/src/components/tooltip/tooltip.tsx

Lines 332 to 343 in 48cade4

	function getTooltipSettings(
	tooltip: TooltipSpec,
	{ externalPointerEvents }: SettingsSpec,
	isExternalTooltipVisible: boolean,
	): TooltipProps {
	if (!isExternalTooltipVisible) return tooltip;
	return {
	...tooltip,
	...externalPointerEvents.tooltip,
	};
	}

...which was returning a new merged instance of the settings for every external event.

Issues

fixes #2309

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• Unit tests have been added or updated to match the most common scenarios