From Xojo Documentation

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

New in 2021r3

Supported for all project types and targets.

The available height (in pixels) of the specified display, taking into account the menubar and/or the taskbar (Dock on macOS), if present. Use the DesktopDisplay.DisplayAt method to call AvailableHeight. For example, DesktopDisplay(0).AvailableHeight returns the available height of the main display.