Answering my own question:
conditions properties determine whether the quick action button appears at all.
conditions evaluate to true, the quick action button appears, but not necessarily the issue content panel.
contentPresentConditions evaluate to true, then the issue content panel will also appear, before you’ve even clicked the quick action button. A ‘hide’ link appears next to the panel title which, when clicked, causes the content panel to collapse, but not disappear entirely. In the collapsed view, only the title of the issue content panel appears, along with a ‘Show’ link. Clicking ‘Show’ causes the panel to expand so that its contents are displayed.
contentPresentConditions evaluate to false, then the issue content panel does not appear initially; and clicking the quick add button makes it appear. A ‘…’ button appears at the top right corner of the issue content panel, which when clicked opens an ‘Actions’ menu that includes a ‘Hide’ link. Clicking ‘Hide’ makes the issue content panel disappear entirely. Clicking the quick action button makes it appear again.
conditions evaluate to false, the quick action button does not appear, and neither does the issue content panel, regardless of the