fix: use continue instead of return in clearStacks loop

In commit 1e296cc, clearStacks was refactored from forEach to
for...of, but the inner `return` (which previously only skipped
the current forEach iteration) now exits the entire function.

This means if any parsed data point lacks _stacks data for the
current dataset (e.g. when two points share x-coordinates and
their _stacks object, or when new points are inserted), the
remaining data points are skipped and their stale stack entries
are never cleared. This causes incorrect stacking when datasets
are reordered.

Fixes #12154

Author: veeceey

src/core/core.datasetController.js

@@ -217,7 +217,7 @@ function clearStacks(meta, items) {
   for (const parsed of items) {
     const stacks = parsed._stacks;
     if (!stacks || stacks[axis] === undefined || stacks[axis][datasetIndex] === undefined) {
-      return;
+      continue;
     }
     delete stacks[axis][datasetIndex];
     if (stacks[axis]._visualValues !== undefined && stacks[axis]._visualValues[datasetIndex] !== undefined) {

test/specs/core.datasetController.tests.js

@@ -952,6 +952,39 @@ describe('Chart.DatasetController', function() {
     expect(meta._parsed[0]._stacks).toEqual(jasmine.objectContaining({y: {0: 10, 1: 20, _top: 1, _bottom: null, _visualValues: {0: 10, 1: 20}}}));
   });
 
+  it('should clear stacks for all data points even when some lack stack data', function() {
+    // Regression test for #12154: clearStacks used `return` instead of
+    // `continue`, so if any parsed item lacked _stacks data the remaining
+    // items were skipped, leaving stale stack entries.
+    var chart = acquireChart({
+      type: 'line',
+      data: {
+        datasets: [{
+          data: [{x: 1, y: 10}, {x: 2, y: 20}, {x: 3, y: 30}],
+          label: 'A'
+        }, {
+          data: [{x: 1, y: 11}, {x: 2, y: 21}, {x: 3, y: 31}],
+          label: 'B'
+        }]
+      },
+      options: {
+        parsing: false,
+        scales: {
+          y: {stacked: 'single'}
+        }
+      }
+    });
+
+    // Swap dataset order
+    var datasets = chart.data.datasets;
+    chart.data.datasets = [datasets[1], datasets[0]];
+
+    // This should not throw or leave stale stack data
+    expect(function() {
+      chart.update();
+    }).not.toThrow();
+  });
+
   describe('resolveDataElementOptions', function() {
     it('should cache options when possible', function() {
       const chart = acquireChart({