Activity overlay
Hold ⇧⌥ and every running application reports what it is costing: processor use above, memory below.
Icons-only cells stack the two figures on a translucent scrim, so the icon still shows through — the question this answers is usually which application is eating the battery, which you cannot read off a covered icon. A labelled button puts the figures where the application’s name would be, and keeps its icon.
Turn it off with Activity Overlay.
Why ⇧⌥
Section titled “Why ⇧⌥”It means nothing else here. ⇧-click quits and ⌥-click hides, but ⇧⌥ together is not a click gesture, so holding it and clicking by accident does nothing. A test asserts that, and will fail if ⇧⌥ ever gains a meaning.
What it samples, and when
Section titled “What it samples, and when”Nothing is sampled until the keys go down, and everything is forgotten when they come up.
Processor use needs two readings to subtract, so the first moment shows — and the figure appears
about half a second later. Memory is exact immediately.
No permission is needed: proc_pid_rusage answers for any process you own, and the modifiers are
read directly rather than monitored.
Helper processes count towards their application
Section titled “Helper processes count towards their application”An Electron application does its work in child processes — VS Code has nine — so reading the application’s own process alone would report a fraction of the truth. Every descendant is added up.
Safari’s tabs are the exception
Section titled “Safari’s tabs are the exception”WebKit’s content processes are started by launchd and have it as their parent, so nothing in the process tree connects them to Safari. macOS knows the real answer — that is what a “responsible process” is — but only exposes it through private API, so Safari reports its own process alone and reads lower than Activity Monitor shows.