With the Artboard tool in use, Ctrl/Cmd+A after a click on a row of the
Artboards panel selected all the art. The panel now takes the keyboard
when one of its rows is clicked, as the Layers panel does for Delete,
and gives it up when the pointer is pressed outside its list or the
panel is not drawn.
While it has the keyboard and the Artboard tool is in use, Select All's
keys, and Select All in the macOS menu bar, select every artboard: the
active artboard stays active and the art's selection does not change.
Otherwise Select All still selects the art.
Docs: docs/target-app-parity.md.