Using the actual monitor width in pixels can lead to unsatisfying
results, depending on the display orientation and physical size: on a
10" tablet with a 1280x800 resolution (scale 1), portrait orientation
will be using the narrow layout.

If the keyboard is sized in an optimal way (i.e. so the layout fills the
whole area, with no blanks on the sides) this would result in an
unnecessarily huge keyboard being displayed, therefore wasting screen
estate.

Using the virtual display size gives a hint about the physical size of
the device, and can be used to select wide layouts even in portrait
mode, while still preserving current behavior on HiDPI devices.

This has been tested on PineTab, PinePhone and Librem 5.