What does it mean that attention residue lingers?

Unfinished tasks leave attention residue that degrades focus on subsequent tasks.

Example: You're writing a proposal when a Slack message pulls you into a hiring discussion. You reply, then return to the proposal — but for the next fifteen minutes your sentences are vague, your reasoning shallow. You're physically back on the proposal, but a piece of your mind is still weighing whether that candidate is worth a second interview. That piece is attention residue, and it is consuming working memory slots you need for the task in front of you.

Try this: The next time you switch tasks, pause for sixty seconds before starting the new one. Write down: (1) where you left off on the previous task, (2) what the next concrete step would be when you return, and (3) any unresolved question that might pull your mind back. This is a ready-to-resume plan. Then start the new task. Notice whether the mental pull toward the old task weakens.