Fix tooltip animation when target changes while animating (chartjs#5005)

* Fix issue chartjs#4989
 - tooltip in 'index' mode doesn't animate smoothly.

* Change: different approach for smooth tooltip animation in 'index'
        mode, when target doesn't change.

* Fix: jslint error

* Fix: remove spyOn pivot from test

* Add: setAnimating-flag in transition used to set on tooltip.transition
     to keep track of tooltip animation.

* Decrease code complexity

* Revert transition and complexity changes
Add: use 'tooltip._start' as workaround check for tooltip animation status

src/core/core.controller.js

@@ -849,7 +849,15 @@ module.exports = function(Chart) {
 			me._bufferedRequest = null;
 
 			var changed = me.handleEvent(e);
-			changed |= tooltip && tooltip.handleEvent(e);
+			// for smooth tooltip animations issue #4989
+			// the tooltip should be the source of change
+			// Animation check workaround:
+			// tooltip._start will be null when tooltip isn't animating
+			if (tooltip) {
+				changed = tooltip._start
+					? tooltip.handleEvent(e)
+					: changed | tooltip.handleEvent(e);
+			}
 
 			plugins.notify(me, 'afterEvent', [e]);
 

src/core/core.tooltip.js

@@ -852,25 +852,19 @@ module.exports = function(Chart) {
 			// Remember Last Actives
 			changed = !helpers.arrayEquals(me._active, me._lastActive);
 
-			// If tooltip didn't change, do not handle the target event
-			if (!changed) {
-				return false;
-			}
-
-			me._lastActive = me._active;
-
-			if (options.enabled || options.custom) {
-				me._eventPosition = {
-					x: e.x,
-					y: e.y
-				};
-
-				var model = me._model;
-				me.update(true);
-				me.pivot();
-
-				// See if our tooltip position changed
-				changed |= (model.x !== me._model.x) || (model.y !== me._model.y);
+			// Only handle target event on tooltip change
+			if (changed) {
+				me._lastActive = me._active;
+
+				if (options.enabled || options.custom) {
+					me._eventPosition = {
+						x: e.x,
+						y: e.y
+					};
+
+					me.update(true);
+					me.pivot();
+				}
 			}
 
 			return changed;