How to activate or restore missing press-and-hold diacritical marks in OS X

A reader has a grave complaint about an acute problem adding accents.
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Glenn Fleishman’s most recent books include Take Control of iOS & iPadOS Privacy and Security, Second Edition (covering iOS 15/iPadOS 15), Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he makes Tiny Type Museums. He’s a senior contributor to Macworld, where he writes Mac 911.