MultiSelect className repeatedly added

[justinbhopper]

Environment

• Package version(s): @blueprintjs/select 3.12.0
• Browser and OS versions: Any

Sandbox illustrating bug is here

Steps to reproduce

1. Create a MultiSelect with tagInputProps.inputProps specified.
2. Focus into the input so that the popover opens.
3. Observe the classnames of the input repeat.

Actual behavior

Classname "bp3-multi-select-tag-input-input" gets repeatedly added to the input.

Expected behavior

Classname "bp3-multi-select-tag-input-input" only gets added once.

Possible solution

This line is the culprit:
https://github.com/palantir/blueprint/blob/develop/packages/select/src/components/select/multiSelect.tsx#L123

Apparently classNames() does not do a comparison of existing classnames when appending the new class to the list. The current code is modifying the existing inputProps, and so this code running multiple times repeatedly adding the classname over and over. I believe this classname has only recently been added, as I did not observe this in version select/3.10.0 (nor did the classname seem to exist back then).