Bad Formatting Due To Unicode U+200B

[korverdev]

Describe the bug

Black adds unexpected indent to docstrings when previous lines contains a zero-width space character.

To Reproduce

Run Black on the following code:

class Test:
    def test(self) -> None:
        """Test.

        Testing...
                ​
        Testing...
​
        Testing...
        """

This results in the following formatting:

class Test:
    def test(self) -> None:
        """Test.

                Testing...
                        ​
                Testing...
        ​
                Testing...
        """

Expected behavior

I would expect this to format as it would without the zero-width space characters, keeping the same indent level as the unformatted code.

Environment

• Black's version: 21.9b0
• OS and Python version: OSX/Python 3.9.7

Additional context

N/A