Goal: pressing ⌘⇧4 should save a region screenshot to ~/Downloads and put the PNG on the clipboard. Out of the box macOS only saves the file (no clipboard). This note captures the debugging journey, the wrong hypotheses, the real root cause, the working fix, and the diagnostic toolkit — so it can be re-debugged or reproduced later.
- You cannot reliably steal ⌘⇧4 from macOS mid-session. The screenshot hotkeys (
com.apple.symbolichotkeys) are registered into the live WindowServer session at login. Disabling one on disk does not take effect until the next login. RestartingSystemUIServerdoes not reload it either. - So the winning design is Plan B — stop fighting the key: let macOS own ⌘⇧4 (it saves the file), and have Hammerspoon watch
~/Downloadsand copy each new screenshot PNG to the clipboard. No logout, no hotkey race, reboot-proof.