OK - so you have this nice crisp screen, and still you want to add extra
monitors to the laptop! That means Blender should switch back and forth to HiDPI
modes, when you move a window to another monitor.
This code makes the pixelsize scale factor a window property, and handles
an event when a window moves to another monitor. It then changes the
native pixelsize nicely and refreshes entire UI.
You can also have one Blender window on high, and other on low resolution.
Stretching a Blender window from 1 monitor to the other works too, but that
is Apple magic handling it.
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