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External monitor connected to Lenovo ThinkPad laptop blacks out randomly

The symptoms: the monitor blanks (turns black) for about half a second before reverting at seemingly random intervals.

The setup: a Lenovo laptop with a default 65W AC adapter power supply (PSU) connected to a wall socket and via HDMI to an external monitor, with the OS configured to run both the built-in and external monitor. According to many forum posts this problem seems to happen for many different Lenovo laptop models, operating systems, and screens, so I’ve omitted those details.