Screen.AvailableLeft
From Xojo Documentation
Read-Only Property (As Integer )
The number of usable pixels from the left edge of the specified screen. This would be zero unless, for example, the user had placed the taskbar (Windows and Linux) or the Dock (macOS) on the left.
Notes
Use the Screen method to call AvailableLeft. For example, Screen(0).AvailableLeft returns the available left number of pixels of the main screen.