Please add expectExceptionMessage to your test (see testNotSerializable in the member you're changing for an example). This helps ensure that the test fails for the "right" reason. I won't insist on it, but it would be helpful if you could do the same for the other tests in this member that call expectException without calling expectExceptionMessage.

Thank you for the feedback, it's done!

I have found a situation which your change does not cover. I need to think about it a little more.

$spreadsheet = new Spreadsheet();
$sheet1 = $spreadsheet->getActiveSheet();
$sheet1->setTitle('firstsheet');
$sheet2 = $spreadsheet->createSheet();
$sheet2->setTitle('Firstsheet', true, false);

If the third parameter to setTitle is true (default), PhpSpreadsheet will append a number to the sheet name until it finds a unique name. However, if it is false, it won't do that, and so this code will create a corrupt spreadsheet. I do not understand the use case for that third parameter. In the test suite, it is used only in WorksheetTest; I would be surprised if it was ever used "in the wild", and I imagine "caveat emptor" should apply to anyone who uses it. So, I don't think you should do anything about it. But, the first statement in Worksheet::setTitle:

        // Is this a 'rename' or not?
        if ($this->getTitle() == $title) {
            return $this;
        }

For consistency with your change, that should probably be a case-insensitive compare as well. Can you make that change and add an appropriate test in WorksheetTest::testSetTitleDuplicate?

No, it turns out that Excel allows you to change the sheet name in this manner (and adjusts defined names etc. accordingly). So the change I suggested to setTitle should not be made after all. Sorry if you wasted any time on it.

Thank you for your contribution.

Thank you for your work!