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When you connect your MacBook to the panel display it defaults to an "extended" workspace. This is a second "desktop" space and your mouse/pointer can slide right off your laptop screen onto the panel screen (usually to the far right or far left edge).
To mirror your MacBook screen with the display instead:
Now whatever is showing on your MacBook screen will also be shown on the display.
Note: the resolution (size of the graphics) may adjust to be compatible with both.