M4 external display shows desktop
M4 ext. display shows desktop but no open windows over HDMI
MacBook Pro 14″, macOS 15.7
Question marked as
Great! That means it working!
From your description, it sounds like it is set to Extended Desktop. not Mirroring.
in Extended Desktop, the displays are concatenated along an edge of your choosing into one Big Extended Desktop. The mouse moves freely across the divide, and can drag a window across to the second display -- or part way across, and each half will be updated "the right way".
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if you wanted "Mirrored", that can be selected instead. When using "Mirrored" BOTH displays are set to a compromise resolution, and are updated simultaneously.