Masterbar shopping cart dropdown no longer appears near cart button

[sirbrillig]

Something changed recently which has caused the masterbar shopping-cart dropdown menu to appear at the top-left of calypso rather than beneath its button.

To reproduce:

1. Visit /plans and add a product to your cart.
2. Leave checkout and return to another calypso page like /home.
3. Click the masterbar shopping-cart icon.

Expected: it would show the cart below the icon.

Actual: it shows the cart off to the side.

[sirbrillig]

I believe this was broken by #98510

Here's what it looks like before that was merged:

[sirbrillig]

With some help by @michaeldcain I believe I found the problem. The ref used by Popover to determine its position is not pointing to an element on the page when it appears. This is because the component that renders the masterbar button is being recreated on every render. Working on a fix.