InputGroup rightElementWidth no longer gets recomputed if rightElement changes

[justinbhopper]

Environment

• Package version(s): @blueprintjs/core 3.23.1
• Browser and OS versions: Windows

Steps to reproduce

1. Create a InputGroup that has no rightElement on mount
2. Update prop of InputGroup to include a rightElement

Actual behavior

The InputGroup does not resize to adjust its padding-right for the rightElement, which causes overlapping.

Expected behavior

The inputGroup recomputes its padding-right to reflect the new rightElement prop.

Possible solution

I believe this bug is caused by a change here, which seemed to be an attempt to fix some sort of infinite loop scenario. The fix ended up removing any re-calculation via componentDidUpdate, which means the right padding is effectively locked in after initial render. I think perhaps we should introduce the componentDidUpdate, but only recompute the rightElementWidth if the rightElement prop changed.

#3813
66b16dd