Fix for Issue 1495 (#1500)

#1495 reports that ActiveSheet can change when calculation
involves jumping around between sheets.
Save index before calculation, restore after, add test.

Author: oleibman

src/PhpSpreadsheet/Cell/Cell.php

@@ -251,9 +251,11 @@ public function getCalculatedValue($resetLog = true)
     {
         if ($this->dataType == DataType::TYPE_FORMULA) {
             try {
+                $index = $this->getWorksheet()->getParent()->getActiveSheetIndex();
                 $result = Calculation::getInstance(
                     $this->getWorksheet()->getParent()
                 )->calculateCellValue($this, $resetLog);
+                $this->getWorksheet()->getParent()->setActiveSheetIndex($index);
                 //    We don't yet handle array returns
                 if (is_array($result)) {
                     while (is_array($result)) {

tests/PhpSpreadsheetTests/Cell/CellTest.php

@@ -46,4 +46,23 @@ public function providerSetValueExplicitException()
     {
         return require 'tests/data/Cell/SetValueExplicitException.php';
     }
+
+    public function testNoChangeToActiveSheet(): void
+    {
+        $spreadsheet = new Spreadsheet();
+        $sheet1 = $spreadsheet->getActiveSheet();
+        $sheet1->setTitle('Sheet 1');
+        $sheet3 = $spreadsheet->createSheet();
+        $sheet3->setTitle('Sheet 3');
+        $sheet1->setCellValue('C1', 123);
+        $sheet1->setCellValue('D1', 124);
+        $sheet3->setCellValue('A1', "='Sheet 1'!C1+'Sheet 1'!D1");
+        $sheet1->setCellValue('A1', "='Sheet 3'!A1");
+        $cell = 'A1';
+        $spreadsheet->setActiveSheetIndex(0);
+        self::assertEquals(0, $spreadsheet->getActiveSheetIndex());
+        $value = $spreadsheet->getActiveSheet()->getCell($cell)->getCalculatedValue();
+        self::assertEquals(0, $spreadsheet->getActiveSheetIndex());
+        self::assertEquals(247, $value);
+    }
 }