Microsoft Wireless Display: black border around image (screen image is scaled down)

windows-10 multiple-monitors graphics-card external-display display-driver

OK I feel stupid: the answer is to open the Intel HD Graphics Control Panel (right click the desktop and choose Intel Graphics Settings, then go to the Wireless Display section and increase the Picture Size to 100%. See screenshot below. You would think this would DEFAULT TO 100% (hello Intel???) but it was defaulted to 75%.

enter image description here

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