The Universal Clipboard Confusion
Apple’s ecosystem relies heavily on a continuity feature known as “Handoff.” This allows your Mac and your iPhone to constantly communicate via Bluetooth and Wi-Fi. When Handoff is active, if you copy a string of text on your iPhone, you can immediately paste it onto your Mac (Universal Clipboard). Furthermore, an icon will appear in your Mac’s dock allowing you to instantly pick up an email or a Safari webpage exactly where you left off on your phone. While magical when working alone, this feature becomes a massive privacy and security issue if you share your Mac with family members, as they can suddenly access what you are doing on your personal phone simply by being in the same room.
Severing the Handoff Connection
To stop your Mac from constantly attempting to sync clipboards and application states with your nearby iPhone, you must completely disable the Handoff protocol on the macOS side.
Open the System Settings app by clicking the Apple logo in the top left corner of your screen. Scroll down the left-hand sidebar and select General. In the main pane on the right, click on AirDrop & Handoff. Locate the toggle switch labelled Allow Handoff between this Mac and your iCloud devices. Click the toggle to turn it Off. The connection will be immediately severed. Your Mac will no longer display the Handoff icons in the dock, and your Universal Clipboard will be disabled, ensuring your iPhone data remains entirely isolated from the shared computer.