From Xojo Documentation

Read-Only Property (As Integer )
IntegerValue = aDesktopDisplay.AvailableWidth

New in 2021r3

Supported for all project types and targets.

The available width (in pixels) of the display, taking into account the taskbar if placed on the left or right of the specified display. Use the DesktopDisplay.DisplayAt method to call AvailableWidth. For example, DesktopDisplay.DisplayAt(0).AvailableWidth returns the available width of the main display.