PanelStack2: Prevent extra render due to updated previous panel

[YoruNoHikage]

Hi,

I had a TextArea component with a jumping cursor from the moment I updated to Blueprint 5 from 4.

This component is located in a second panel within a PanelStack2 but the first one had changing props at every render.
When digging through the differences between v4 and v5, I found out that handleClose has been added to the useMemo dependencies of PanelWrapper triggering a render every time previousPanel changes.

So for now, to go around this, I wrap the previous panel with a useMemo as well.

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Extracting previousPanel !== undefined outside the callback prevent extra renders by using a boolean instead.

[palantirtech]

Thanks for your interest in palantir/blueprint, @YoruNoHikage! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.