[core] fix(TextArea): resize vertically in controlled mode

[AaronJRubin]

Fixes #5828

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Resize text area when its value changes due to a new string being passed in through props

Reviewers should focus on:

How we can test this - at first, I tried adding a test like the below:

    it("resizes when props change if growVertically is true", () => {
        const initialText = "A";
        const longText = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
        const wrapper = mount(<TextArea growVertically={true} value={initialText} style={{ marginTop: 10 }} />);
        const textarea = wrapper.find("textarea");
        const initialHeight = (textarea.getDOMNode() as HTMLElement).style.height;
        wrapper.setProps({ value: longText });
        const newHeight = (textarea.getDOMNode() as HTMLElement).style.height;
        assert.notEqual(newHeight, initialHeight);
    });

But both newHeight and initialHeight are always 0, so this test fails. The existing test for the resize behavior (can fit large initial content) appears to be spurious; the comparison at https://github.com/palantir/blueprint/blob/develop/packages/core/test/forms/textAreaTests.tsx#L67 is also just comparing the strings 0px and 0px.

I manually tested this by building another app against the locally-compiled blueprint-core package and observing that the buggy behavior was fixed, but I'm not sure if that meets our standards for testing.

[adidahiya]

@AaronJRubin I think the tests are not working because we're not mounting into an attachTo container element. I'll poke at the test suite in this PR

[adidahiya]

@AaronJRubin I wasn't able to get the tests working, but I've added a little example to the docs which demonstrates controlled usage w/ growVertically. it seems to work with your change. I went ahead and implemented some code style fixes / improvements on top of your commit.

Note that it does not shrink vertically automatically, but this matches the existing behavior with the uncontrolled API.

[adidahiya]

self-review

[adidahiya]

this doesn't really help us. removed this comment

[adidahiya]

I don't see a significantly better way to do this, and I think the potential for layout thrashing is minimal. removed this comment.

[adidahiya]

note that props.style changes can cause layout changes which affect scrollHeight